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I read this article today. I understood a few things, but few remained unanswered.

The article mentioned 18 basic colors. The things little confused me after that line. I assumed Pantone have only 18 spot colors currently.

So 18 basic colors are spot colors.

Okay, that's about spot colors.

Now, in process colors, I understand so far that colors produced while printing by using C, M, Y and K inks are called process colors.

Also, the article mentioned that mixing of colors produce spot colors (if I understand correctly). So, when CMYK is also a mixing of colors and results in new colors, I wonder why the colors aren't called spot colors.

Given all that, it may seem a lot of doubts, but the ultimate aim is to know the true meaning of spot colors and why CMYK mix is not same as spot color mix.

Simply:

CMYK is not mixed. A printing process generally do not mix colors. It is kind of the holy grail of printing processes but it does not really exist as of yet.

What happens instead is that printing processes layer transparent inks on top of each other. Each ink only being able to produce either full ink color or no ink color. The indeterminate colors are achieved by alternating these cells of of ink or no ink on top of each other. Result is what looks like some value between the colors and white but is no where as rich as a mixed ink. We call a super cell of printer dots that produces a apparent color we call a print raster.

Spot colors on the other hand are mixed. So if you want lime green no problem lets mix lime green. You can again not print different mixtures, just rasterize different spots or process colors together but if all you want is lime green then you get it exactly as it is.

To understand the biggest difference between the two you would have to work with retail packaging. See in retail you have packages form different batches, possibly form different factories side by side. Now if there is a small but discernible color variation between these batches people would act as if the other batch is somehow tainted, they would gravitate towards the other batch, this leads to a lot of lost sales.

Now process color is more fragile when it comes to reproduction than spot colors. Mainly because reflected surfaces react with ambient light. The inks have some color spectra, humans of course do not see spectra. However when the surrounding light around reacts with the different inks something special can happen. They can under some lights seem the same and different when seen in other light condition. So while CMYK can reproduce same color under same light a spot color can reproduce the same physics as the other if substrate is same. Also if you are willing to mix custom proportions you can possibly get same color on other substrates.

You also get a cleaner color, and a wider gamut if you are willing to mix colors. You also get more even surfaces of same color and possibly gradients too. Also it is possible to dye plastics or fabrics with some spot systems so then you can order material in your exacting color, none of which you can easily do with process colors.

Alright a typical printer has 4 cartridges, CMYK. It looks kinda like this with the black cartridge usually a bit larger because it gets used more frequently:

If you print on this printer and the image has green in it then its going to put tiny dots on the paper of both Cyan and Yellow in order to make that green. This is what CMYK process is.

Pantone and Spot Colors in general can be thought of almost like this. You have your CMYK and then you have a pre-mixed precise color. Say its a Green spot color. Now you have a precise green and don't need to mix Cyan and Yellow to attempt to get the right green. You just put down the already mixed green.

This is very expensive and while I have no stats to support it would venture to say most print shops cannot even do spot colors like this.

Let's pretend your Coca-Cola and typically always use a very precise spot color for your red. But if you're Coca-Cola you may at times be using a printer that doesn't do spot colors. Internal memos likely are printed on basic office printers. In that case they approximate their signature red using as close a CMYK equivalent as they can.

Another key thing is some colors cannot be produced at all within the CMYK space. Neon colors for example require the use of a neon pigments. This would be another example of a spot color. See Alan's answer here: https://graphicdesign.stackexchange.com/a/4473/2611

Now regarding the 18 base colors it talks about. They use 18 base paints, which they mix in industrial mixers to precise ratios, and then that mixture becomes for example Pantone 1925C.

Process or CMYK colors are, as you've seen, colors mixed from C, M, Y and K inks.

Spot colors are generally printed using a specific color ink mixed from one of the 18 Pantone inks that you mentioned, though sometimes printers use a formula that allows them to mix a spot color (or an approximation of it) using CMYK inks.

TLDR: No, spot colours are not limited, and they aren't treated the same because they are different printing processes.

Process printing (or CMYK printing, also sometimes called full-colour printing) uses separate inks. They aren't mixed.

The inks are layered onto the paper, usually using halftone dots to build up a pattern of dots to simulate a solid colour when viewed at normal viewing distances. In offset lithographic printing, this usually means using a press with four print heads, and four printing plates, one for each colour, or sometimes using one or two heads with multiple passes through a printing press.

A typical 4 colour process offset lithographic printing press, showing the four colour heads (CMYK), and ink ducts, which supply ink to the rollers, and subsequently to the 4 printing plates, and the image is transferred to a rubber blanket, and then onto a sheet of paper under compression between the blanket and a steel cylinder.

Spot colours are printed using one single ink, often made by physically mixing different base colour inks to a formula from a Pantone guide book - a bit like the same way you'd mix some base paint colours to get a new colour. These are printed using only one printing head, and one plate, and typically with solid areas of ink, although it's possible to use them with halftones too. Spot colours have a wider gamut than CMYK colours, and there are spot colours which have no equivalent in CMYK - for example, metallic colours, or fluorescent colours, or inks with pure pigments such Reflex Blue, which is notoriously difficult to reproduce in CMYK

Sometimes very large printing presses have more than 4 print heads, allowing the extra ones to be used for spot colours.

You will likely need to use a magnifying glass to see it, but this is what a printed process colour looks like compared to a solid spot colour close up.