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Tinge

1 : a slight staining or suffusing shade or color

The Northern Lights neon green with a tinge of purple.

The word tint also has validity here.

1a : a usually slight or pale coloration : HUE

The Northern Lights are neon green with the slightest purple tint to them.

Both of these words have the advantage of being associated with color specifically.

You can also use hint.

3 : a very small amount : SUGGESTION

The Northern Lights are neon green with a hint of purple fringing.

I like dab for this context, because it's associated with paint as both a noun and a verb:

dab noun

1: a sudden blow or thrust : POKE
2: a small amount

3: a gentle touch or stroke : PAT

4: DAUB

dab verb
dabbed; dabbing

1: to strike or touch lightly : PAT

//dabbing her eyes with a handkerchief
2: to apply lightly or irregularly : DAUB

//dab paint
_{from m-w.com}

Further,

A dab is a quick, soft application of something, like paint or makeup. [...]
A dab is the motion with which you pat or apply something to a surface, and it's also the tiny amount of material you're applying. An artist might work in dabs of paint...
_{from vocabulary.com}

I would use "trace" as in

The northern lights are neon green mixed with traces of purple.

The OED defines trace as:

6c. An indication of the presence of a minute amount of some constituent in a compound; a quantity so minute as to be inferred but not actually measured;

This is the first definition given for the noun 'trace' in https://www.merriam-webster.com/dictionary/trace, the first definition of the word as a noun in https://www.collinsdictionary.com/dictionary/english/trace

The northern lights are neon green mixed with a touch of purple.

From https://www.merriam-webster.com/dictionary/a%20touch%20of:

a touch of (idiom):

a small amount of (something) : a hint or trace of (something)

The northern lights are neon green mixed with a splash of purple.

splash:

• a small quantity of liquid that has fallen or been dashed against a surface.
  
  
  • "a splash of gravy"
  
  
  • Similar: spot, blob, dab, daub, smudge, smear, speck, speckle, fleck, patch, pop, mark, stain, splotch, splosh, splodge
  
  


• a small quantity of liquid added to a drink.
  
  
  • "a splash of lemonade"
  
  
  • Similar: drop, dash, bit, spot, soupçon, dribble, driblet, little, small amount, scoosh
  
  


• a bright patch of color.
  
  
  • "add a red scarf to give a splash of color"
  
  
  • Similar: patch, burst, streak
  
  

Smattering

The second definition from the linked page:
2: a small scattered number or amount

In your example with the aurora it could be used like so:

Largely neon green with a smattering of purple around the edges.

Be aware that this word only fits if the color is somewhat scattered.

Because the second colour isn't a uniform tone, how about:

• "neon green with flashes of purple" - the purple isn't something that affects the entire image, but is only present in small areas.

Because the purple is predominantly outside the green areas, consider:

• "neon green, fringed with purple" - using the verb is nicer than just saying "with a purple fringe")


• "neon green with a purple aura" - prior to its "New Age" connotations, one of the meanings of Aura was simply "luminous radiation".

Not a native English speaker, but how about a tad?

https://en.wiktionary.org/wiki/tad#English

Noun

tad (plural tads)

(informal) A small amount; a little bit.

 Could you lean the picture to the left just a tad more?

Along the same lines as some of the other suggestions, a "dash" would work as well.

The northern lights are neon green mixed with a dash of purple.

Dash is another word commonly used in cooking, similar to pinch or smidgen, meaning a small amount.

Since it's the Northern lights, I would be inclined to say something like "folds/ribbons/threads of purple" (it is often described with curtain similes, you see.) I also wouldn't use the word "mixed" in this context either. Try "blended", "woven", "laced". In fact, "The Northern lights are neon green, laced/streaked with purple" would be an option. Verbs are always better.

"A light shade of"

The northern lights are neon green mixed with a light shade of purple.

Definition of shade from https://www.merriam-webster.com/dictionary/shade

8b: a color slightly different from the one under consideration

I would use the term "a pinch" as in

The northern lights are neon green mixed with a pinch of purple.

in this context "pinch" would be a metaphorical use of the method of mixing color powders (like makeups) by pinching a small quantity of one color and adding it into a larger pile of a different colors

Besides all the words already recommended, here are a few more colored pixels to consider (Some just synonyms for earlier suggestions):

A bit of purple. A wisp. A mere suggestion. An echo. A twinkle. A dusting. A dappling.

When it comes to small amounts of color in visual descriptions, almost any word that suggests small/short, brief/rapid, ephemeral/transitory can be used. There must be, literally, thousands.

But don't stop there. You can go full metaphorical: Green with a vibration of purple. Green foam on a purple sea. Purple sparkles on green gemstones. An opium haze floating over billowing jade curtains.