I want to make an arrow that has gradual color as shown in the picture below:

First I wanted to make that arrow in one stroke, but I do not know how to make shaded color in postaction, so in the end I draw right side first, left side second and put shade over it in the middle.

Now two problems appear.

1. Color shading is not perfect, the middle colors do not correspond well to colors on the left and on the right. I found out that the problem comes from the fact I opened xcolor in cmyk mode.


2. The thickness of the colored part of the arrow is not perfectly constant. This cannot be seen on the picture but on the MWE-created PDF.

Is there any other way to create such effect, let's say in one stroke? If not, how can colors created by draw be aligned with colors created by shade?

IMPORTANT EDIT: I would prefer a tweak to the existing solution, if possible. Namely I am already using dozens of such arrows without color gradient going between two specified points following a bent path, i.e. (0,0) -- (0.5,0) arc (-90:0:0.5) -- (1,1). Now I just need ONE straight but with gradient color.

MWE:

documentclassminimal

usepackage[cmyk]xcolor
usepackagetikz
usetikzlibraryarrows.meta

begindocument

begintikzpicture

draw[line width=4.15mm,Triangle Cap[reversed,cap angle=120]-,postaction=draw,color=red!75,line width=3.85mm,shorten <=0.25mm] (0,0) -- (-0.8,0);
draw[line width=4.15mm,-Triangle Cap[cap angle=120],postaction=draw,color=red!25,line width=3.85mm,shorten >=0.21mm] (-0.8,0) -- (-1.6,0);
shade[left color=red!25,right color=red!75] (-0.6,-0.5*3.85mm) rectangle (-1.0,0.5*3.85mm);

endtikzpicture
enddocument

The shapes library has a special shape called "signal". Using this one can shade the whole shape in one go:

documentclass[margin=3mm]standalone

usepackage[cmyk]xcolor
usepackagetikz
usetikzlibraryarrows.meta,shapes

begindocument

begintikzpicture
node[signal, draw,minimum width=1cm, signal from=east, signal to=west,shade, right color=red!75, left color=red!25] at (0,1) ;
endtikzpicture

enddocument

This combines the shape from this answer with some of the aspects of this answer, where a similar problem was addressed. (However, this uses show path construction instead of a to path because this is arguably slightly easier to deal with.)

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarycalc,shapes.symbols,decorations.pathreplacing

begindocument

begintikzpicture[
 signal arrow/.style=decorate,decoration=show path construction, 
 lineto code=
 path let p1 = ($(tikzinputsegmentlast)-(tikzinputsegmentfirst)$),
 n1 = int(mod(scalar(atan2(y1,x1))+360, 360)),
 n2 = veclen(x1,y1)
 in
 (tikzinputsegmentfirst) -- (tikzinputsegmentlast)
 node[signal,midway,sloped,left color=red,right color=red!20,draw,
 signal from=west, signal to=east,minimum width=n2-pgflinewidth,
 inner xsep=0pt,inner ysep=5pt,shading angle=n1+90,
 anchor=center,#1];
 ,
 signal arrow/.default= % empty default
]
 path[signal arrow] (0,0) to (1,2);
 path (3,0) node[circle,inner sep=2pt,fill] (A) 
 (5,3) node[circle,inner sep=2pt,fill] (B) ;
 path[signal arrow] (A) to (B);
endtikzpicture

enddocument

Just as proof of principle: a minor modification of this post. At this point I do not have enough motivation to polish this. This will make more sense IMHO when it is clear where the journey will go.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarydecorations.markings,calc
newcounterparrow
begindocument
begintikzpicture[record path/.style=/utils/exec=tikzsetparrow/.cd,#1,
 decorate,decoration=markings,mark=at position 0 with
 setcounterparrow1%typeoutpgfdecoratedpathlength
 path (0,pgfkeysvalueof/tikz/parrow/dist/2) coordinate (parrowt-pgfkeysvalueof/tikz/parrow/name-numbervalueparrow)
 (0,-pgfkeysvalueof/tikz/parrow/dist/2)coordinate (parrowb-pgfkeysvalueof/tikz/parrow/name-numbervalueparrow);
 pgfmathsetmacromystep(pgfdecoratedpathlength-4pt)/int(1+(pgfdecoratedpathlength-4pt)/2pt)
 xdefmystepmystep,
 mark=between positions 2pt and 1 step mystep pt with stepcounterparrow%
 path (0,pgfkeysvalueof/tikz/parrow/dist/2) coordinate (parrowt-pgfkeysvalueof/tikz/parrow/name-numbervalueparrow)
 (0,-pgfkeysvalueof/tikz/parrow/dist/2)coordinate (parrowb-pgfkeysvalueof/tikz/parrow/name-numbervalueparrow)
 (0,0) coordinate (parrowm-pgfkeysvalueof/tikz/parrow/name-numbervalueparrow);
 ,reconstruct top/.style=insert path=plot[variable=t,samples at=1,...,numbervalueparrow,smooth] (parrowt-#1-t),
 reconstruct bottom/.style=insert path=plot[variable=t,samples at=numbervalueparrow,thenumexprvalueparrow-1,...,1,smooth]
 (parrowb-#1-t),
 font=sffamily,nodes=circle,draw,parrow area/.style=insert path=
 (parrowt-#1-1) [reconstruct top=#1] -- (parrowb-#1-numbervalueparrow) 
 [reconstruct bottom=#1] -- (parrowt-#1-1),
 parrow/.cd,dist/.initial=3.14pt,step/.initial=2pt,name/.initial=]
 path (0,0) node (A) A (3,1) node (B) B ;
 path[record path=name=AB,dist=5pt] (A) to[bend left] (B);
 addtocounterparrow-2
 draw let p1=($(B)-(A)$),n1=atan2(y1,x1) in [left color=red,right
 color=red!20,shading angle=n1+90]
 [reconstruct top=AB] -- (parrowm-AB-thenumexprvalueparrow+2)
 -- (parrowb-AB-numbervalueparrow)
 [reconstruct bottom=AB]
 -- (parrowm-AB-3) -- (parrowt-AB-1);
endtikzpicture
enddocument

Second approach, with postaction:

With the third version of the answer 1 is also possible to achieve the same result with postaction:

Output shading1:

Output shading2:

Code:

documentclassarticle

usepackage[cmyk]xcolor
usepackagetikz
usetikzlibraryfadings,decorations.pathmorphing,arrows.meta,shapes

%https://tex.stackexchange.com/questions/137357/how-to-draw-an-arrow-with-two-colors
makeatletter
newififtikz@shading@path

tikzset
 % There are three circumstances in which the fading sep is needed:
 % 1. Arrows which do not update the bounding box (which is most of them).
 % 2. Line caps/joins and mitres that extend outside the natural bounding 
 % box of the path (these are not calculated by PGF).
 % 3. Other reasons that haven't been anticipated.
 shading xsep/.store in=tikz@pathshadingxsep,
 shading ysep/.store in=tikz@pathshadingysep,
 shading sep/.style=shading xsep=#1, shading ysep=#1,
 shading sep=0.0cm,


deftikz@shadepath#1% 
 % tikz@addmode installs the `modes' (e.g., fill, draw, shade) 
 % to be applied to the path. It isn't usualy for doing more
 % changes to the path's construction.
 iftikz@shading@path%
 else%
 tikz@shading@pathtrue%
 % Get the current path.
 pgfgetpathtikz@currentshadingpath%
 % Get the shading sep without setting any other keys.
 begingroup%
 pgfsys@beginscope% <- may not be necessary
 tikzset#1%
 xdeftikz@tmpnoexpanddefnoexpandtikz@pathshadingxseptikz@pathshadingxsep%
 noexpanddefnoexpandtikz@pathshadingyseptikz@pathshadingysep%
 pgfsys@endscope%
 endgroup
 tikz@tmp%
 % Get the boudning box of the current path size including the shading sep
 pgfextract@processpgf@shadingpath@southwestpgfpointaddpgfqpointpgf@pathminxpgf@pathminy%
 pgfpoint-tikz@pathshadingxsep-tikz@pathshadingysep%%
 pgfextract@processpgf@shadingpath@northeastpgfpointaddpgfqpointpgf@pathmaxxpgf@pathmaxy%
 pgfpointtikz@pathshadingxseptikz@pathshadingysep%
 % Clear the path
 pgfsetpathpgfutil@empty% 
 % Save the current drawing mode and options.
 lettikz@options@saved=tikz@options%
 lettikz@mode@saved=tikz@mode%
 lettikz@options=pgfutil@empty%
 lettikz@mode=pgfutil@empty%
 % tikz@options are processed later on.
 tikz@addoption%
 pgfinterruptpath%
 pgfinterruptpicture%
 begintikzfadingfrompicture[name=.]
 pgfscope%
 tikzsetshade path/.style=% Make absolutely sure shade path is not inherited.
 path pgfextra%
 % Set the softpath. Any transformations,draw=none in #1 will have no effect.
 % This will *not* update the bounding box...
 pgfsetpathtikz@currentshadingpath%
 % ...so it is done manually.
 pgf@shadingpath@southwest
 expandafterpgf@protocolsizesthepgf@xthepgf@y%
 pgf@shadingpath@northeast%
 expandafterpgf@protocolsizesthepgf@xthepgf@y%
 % Install the drawing modes and options.
 lettikz@options=tikz@options@saved%
 lettikz@mode=tikz@mode@saved%
 ;
 % Now get the bounding box of the picture.
 xdefpgf@shadingboundingbox@southwestnoexpandpgfqpointthepgf@picminxthepgf@picminy%
 xdefpgf@shadingboundingbox@northeastnoexpandpgfqpointthepgf@picmaxxthepgf@picmaxy%
 endpgfscope
 endtikzfadingfrompicture%
 endpgfinterruptpicture%
 endpgfinterruptpath%
 % Install a rectangle that covers the shaded/faded path picture.
 pgftransformreset%
 pgfpathrectanglecornerspgf@shadingboundingbox@southwestpgf@shadingboundingbox@northeast%
 %
 % Reset all modes.
 lettikz@path@picture=pgfutil@empty%
 tikz@mode@fillfalse%
 tikz@mode@drawfalse%
 %tikz@mode@tipsfalse% <- To have successful compilation with pgf-tikz v3.0.1a
 tikz@mode@doublefalse%
 tikz@mode@clipfalse%
 tikz@mode@boundaryfalse%
 tikz@mode@fade@pathfalse%
 tikz@mode@fade@scopefalse%
 % Now install shading options.
 tikzset#1%
 tikz@mode%
 % Make the fading happen.
 deftikz@path@fading.%
 tikz@mode@fade@pathtrue%
 tikz@fade@adjustfalse%
 % Shift the fading to the mid point of the rectangle
 pgfpointscale0.5pgfpointaddpgf@shadingboundingbox@southwestpgf@shadingboundingbox@northeast%
 edeftikz@fade@transformshift=(thepgf@x,thepgf@y)%
 pgfsetfadingtikz@path@fadingtikz@do@fade@transform%
 tikz@mode@fade@pathfalse% 
 %
 fi%
}
tikzset
 shade path/.code=%
 tikz@addmodetikz@shadepath#1%
 

makeatother % <- To close the makeatletter call
%-------------------------------------------------------------------------------------
%https://tex.stackexchange.com/questions/197793/how-to-draw-gradient-arrows-with-tikz
makeatletter
defcreateshadingfromlist#1#2#3%
 pgfutil@tempcnta=0relax
 pgfutil@forpgf@tmp:=#3doadvancepgfutil@tempcnta by1%
 ifnumpgfutil@tempcnta=1relax%
 edefpgf@speccolor(0)=(#3);color(100)=(#3)%
 else%
 pgfmathparse50/(pgfutil@tempcnta-1)letpgf@step=pgfmathresult%
 %
 pgfutil@tempcntb=1relax%
 pgfutil@forpgf@tmp:=#3do%
 ifnumpgfutil@tempcntb=1relax%
 edefpgf@speccolor(0)=(pgf@tmp);color(25)=(pgf@tmp)%
 else%
 ifnumpgfutil@tempcntb<pgfutil@tempcntarelax%
 pgfmathparse25+pgf@step/4+(pgfutil@tempcntb-1)*pgf@step%
 edefpgf@specpgf@spec;color(pgfmathresult)=(pgf@tmp)%
 else%
 edefpgf@specpgf@spec;color(75)=(pgf@tmp);color(100)=(pgf@tmp)%
 fi%
 fi%
 advancepgfutil@tempcntb by1relax%
 %
 fi%
 csname pgfdeclare#2shadingendcsname#1100pgf@spec%

makeatother

createshadingfromlistshading1verticalred,white
createshadingfromlistshading1bhorizontalred,white
createshadingfromlistshading2verticalred,yellow,green,cyan,blue
createshadingfromlistshading2bhorizontalred,yellow,green,cyan,blue
begindocument


begintikzpicture
draw[draw,line width=4.15mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],postaction=draw=white, shade path=shading xsep=0.6cm, shading=shading1, line width=3.85mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],shorten <=0.25mm,shorten >=0.21mm] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
endtikzpicture
begintikzpicture
draw[draw,line width=4.15mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],postaction=draw=white, shade path=shading xsep=0cm, shading=shading1b,line width=3.85mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],shorten <=0.25mm,shorten >=0.21mm] (0,0) -- (1.6,0);
endtikzpicture
begintikzpicture
draw[draw,line width=4.15mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],postaction=draw=white, shade path=shading xsep=0cm, shading=shading1b,line width=3.85mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],shorten <=0.25mm,shorten >=0.21mm] (0,0) -- ++(15:1) -- +(45:1);
endtikzpicture
begintikzpicture
draw[draw,line width=4.15mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],postaction=draw=white, shade path=shading xsep=0cm, shading=shading1b,line width=3.85mm,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],shorten <=0.25mm,shorten >=0.21mm] (0,0) arc (180:50:2);
endtikzpicture

enddocument

First approach, with "overlapping":

Based on this answer to tikz: draw multicolor curve with smooth gradient, which relies on other two (1,2) given by Mark Wibrow, you can also achieve a color gradient with the shaping path and the commands of the Shading library (Chapter 69, p.737): left color=<color> and right color=<color> (or top color=<color> and bottom color=<color>).

For a multicolor gradient, you can define:

createshadingfromlist<name><direction><colors>

Output shading1:

Output shading2:

Code:

documentclassarticle


usepackage[cmyk]xcolor
usepackagetikz
usetikzlibrarydecorations.markings,fadings,arrows.meta

%https://tex.stackexchange.com/questions/137357/how-to-draw-an-arrow-with-two-colors
makeatletter
newififtikz@shading@path

tikzset
 % There are three circumstances in which the fading sep is needed:
 % 1. Arrows which do not update the bounding box (which is most of them).
 % 2. Line caps/joins and mitres that extend outside the natural bounding 
 % box of the path (these are not calculated by PGF).
 % 3. Other reasons that haven't been anticipated.
 fading xsep/.store in=pgfpathfadingxsep,
 fading ysep/.store in=pgfpathfadingysep,
 fading sep/.style=fading xsep=#1, fading ysep=#1,
 fading sep=0.0cm,
 shading path/.code=%
 % Prevent this stuff happning recursively.
 iftikz@shading@path%
 else%
 tikz@shading@pathtrue%
 % tikz@addmode installs the `modes' (e.g., fill, draw, shade) 
 % to be applied to the path. It isn't usualy for doing more
 % changes to the path's construction.
 tikz@addmode%
 pgfgetpathpgf@currentfadingpath%
 % Get the boudning box of the current path size including the fading sep
 pgfextract@processpgf@fadingpath@southwestpgfpointaddpgfqpointpgf@pathminxpgf@pathminy%
 pgfpoint-pgfpathfadingxsep-pgfpathfadingysep%%
 pgfextract@processpgf@fadingpath@northeastpgfpointaddpgfqpointpgf@pathmaxxpgf@pathmaxy%
 pgfpointpgfpathfadingxseppgfpathfadingysep%
 % Clear the path
 pgfsetpathpgfutil@empty% 
 % Interrupt the path and picture to create a fading.
 pgfinterruptpath%
 pgfinterruptpicture%
 begintikzfadingfrompicture[name=.]
 path [shade=none,fill=none, #1] pgfextra%
 % Set the softpath. Any transformations in #1 will have no effect.
 % This will *not* update the bounding box...
 pgfsetpathpgf@currentfadingpath%
 % ...so it is done manually.
 pgf@fadingpath@southwest
 expandafterpgf@protocolsizesthepgf@xthepgf@y%
 pgf@fadingpath@northeast%
 expandafterpgf@protocolsizesthepgf@xthepgf@y%
 ;
 % Now get the bounding of the picture.
 xdefpgf@fadingboundingbox@southwestnoexpandpgfqpointthepgf@picminxthepgf@picminy%
 xdefpgf@fadingboundingbox@northeastnoexpandpgfqpointthepgf@picmaxxthepgf@picmaxy%
 %
 endtikzfadingfrompicture%
 endpgfinterruptpicture%
 endpgfinterruptpath%
 % Install a rectangle that covers the shaded/faded path picture. 
 pgfpathrectanglecornerspgf@fadingboundingbox@southwestpgf@fadingboundingbox@northeast%
 % Make the fading happen.
 deftikz@path@fading.%
 tikz@mode@fade@pathtrue%
 tikz@fade@adjustfalse%10pt
 % Shift the fading to the mid point of the rectangle
 pgfpointscale0.5pgfpointaddpgf@fadingboundingbox@southwestpgf@fadingboundingbox@northeast%
 edeftikz@fade@transformshift=(thepgf@x,thepgf@y)%
 %
 fi%
 

makeatother
%-------------------------------------------------------------------------------------
%https://tex.stackexchange.com/questions/197793/how-to-draw-gradient-arrows-with-tikz
makeatletter
defcreateshadingfromlist#1#2#3%
 pgfutil@tempcnta=0relax
 pgfutil@forpgf@tmp:=#3doadvancepgfutil@tempcnta by1%
 ifnumpgfutil@tempcnta=1relax%
 edefpgf@speccolor(0)=(#3);color(100)=(#3)%
 else%
 pgfmathparse50/(pgfutil@tempcnta-1)letpgf@step=pgfmathresult%
 %
 pgfutil@tempcntb=1relax%
 pgfutil@forpgf@tmp:=#3do%
 ifnumpgfutil@tempcntb=1relax%
 edefpgf@speccolor(0)=(pgf@tmp);color(25)=(pgf@tmp)%
 else%
 ifnumpgfutil@tempcntb<pgfutil@tempcntarelax%
 pgfmathparse25+pgf@step/4+(pgfutil@tempcntb-1)*pgf@step%
 edefpgf@specpgf@spec;color(pgfmathresult)=(pgf@tmp)%
 else%
 edefpgf@specpgf@spec;color(75)=(pgf@tmp);color(100)=(pgf@tmp)%
 fi%
 fi%
 advancepgfutil@tempcntb by1relax%
 %
 fi%
 csname pgfdeclare#2shadingendcsname#1100pgf@spec%

makeatother

createshadingfromlistshading0verticalblack
createshadingfromlistshading1verticalred,white
createshadingfromlistshading2verticalred,yellow,green,cyan,blue

begindocument
begintikzpicture
path[shading=shading0,shading path=draw=transparent!0,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],line width=4.15mm,] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
path[shading=shading1,shading path=draw=transparent!0,Triangle Cap[reversed,cap angle=120]-Triangle Cap[cap angle=120],line width=3.85mm,shorten <=0.25mm,shorten >=0.21mm] (0,0) -- (0.5,0) arc (0:90:0.5) -- (1,1);
endtikzpicture

enddocument

TODO:

• Must: the line color has to be black.


• Optional: the color gradient has to follow the direction of the arrow