This is an introductory look into the main colour models used in digital art and how they relate to printing and traditional mediums.
Additive Colour Model
The RGB colour model is an additive colour model in that the primary colours of light: red, green and blue are added together to produce a broad array of colours according to a colour display spectrum. When red, green and blue overlap each other at full intensity they produce white.
When they are all together at zero intensity they produce black.
Incidentally because most art software uses the RGB colour model, the colour in digital RGB does not act the same way pigments in traditional art do.
Subtractive Colour Model
Pigment based colour models use the subtractive colour model.
It is perhaps useful to know that digital painting has not generally used this model for mixing colours as it is based on the additive colour model of RGB light which is very linear in nature, hence why you get what are called linear gradients.
Colour mixing when painting digitally does not behave the same way as traditional paint although some software strives to emulate it because there are other factors at play in the real world and it all gets pretty complicated. You will have to look into the physics of light and things like that.
The Kubelka and Munk model is useful in this regard though I’m not going to pretend I understand it all. Interestingly it is being used for innovative digital colour mixing solutions such as Mixbox and has even been incorporated into the latest version of Rebelle. I’ll be writing more about this in a future post.
The traditional colour mixing theory used the RYB colour model which represents the primary colours red, yellow and blue on the colour wheel. This is the method I remember being taught in early art lessons when I was a kid. These colours used to be believed capable of mixing all the other colours, becoming the foundation of 18th century theories of colour vision.
Theories about colour have since moved on with the Munsell colour system being a more updated way of learning about how colour works than the RYB colour model.
James Gurney wrote a really great series of posts about colour theory on his blog if you want to dig deeper.
CMY and CMYK
CMY represents Cyan, Magenta and Yellow. The are the main primaries in colour printing.
Purely photographic color processes almost never include a K component, because in all common processes the CMY dyes used are much more perfectly transparent, there are no registration errors to camouflage, and substituting a black dye for a saturated CMY combination, a trivial prospective cost-benefit at best, is technologically impractical in non-electronic analog photography.https://en.wikipedia.org/wiki/Subtractive_color
Black (K) as a colour is introduced for inkjet printing and mass produced photo-mechanical printing processes resulting in the CMYK colour model.
The black ink serves to cover unwanted tints in dark areas of the printed image, which result from the imperfect transparency of commercially practical CMY inks; to improve image sharpness, which tends to be degraded by imperfect registration of the three color elements; and to reduce or eliminate consumption of the more expensive color inks where only black or gray is required.https://en.wikipedia.org/wiki/Subtractive_color
If you are reading this you may be asking yourself, since the method of colour mixing in digital art uses the RGB colour model why do we need to be aware of the CMYK model and how can we check colours will look as intended?
If your art is going to be printed using a CMYK printer you have to bear in mind that the colours on your RGB display will not all be reproduced correctly for CMYK printing due to a combination of opposite colour models being used with additive for RGB and subtractive for CMYK as well as the inks in CMYK colour printing having a different colour gamut to your screen.
Light, saturated colors often cannot be created with CMYK, and light colors in general may make visible the halftone pattern. Using a CcMmYK process, with the addition of light cyan and magenta inks to CMYK, can solve these problems, and such a process is used by many inkjet printers, including desktop models.https://en.wikipedia.org/wiki/CMYK_color_model
When designing colours intended for CMYK printing it is wise to be aware of this and you can use CMYK proofing methods with ICC profiles. These are usually available in most modern digital art software.
A common solution for working with the colours on your screen is to design in RGB and then convert your project to CMYK before sending it to the printers. It depends on how good your printer is at representing greys and blacks.
If the printer is only using CMY inks then working in CMYK proofing mode is advisable. I remember my sister telling me years ago that she had to make sure there was no black or K in her colours for the printing company that printed the colour comics she was working on, but things may have changed since.
As to how to convert RGB to CMYK this depends on the software you are using and whether it can do it in the first place and whether you have the CMYK ICC profile from your printer in order to do it correctly. Adobe Photoshop definitely can convert to CMYK and I believe Krita can as well. It may be easier just to design in RGB and then let your printer handle the conversion though since they will know exactly how to handle it.
Colours in CMYK will not look exactly the same as on your screen in RGB because generally they use a smaller colour gamut, this is why proofing is advisable.
Because technology is advancing and some printers can represent many more colours than they used to be able to. As well as this innovative modern RGB printing methods are becoming available such as Spectraval, but unless your printer is using such methods it is probably wise to assume they are using the CMYK method.
If you are designing art purely for screens and the web you don’t have to worry about the CMYK colour model, but you do have to look into sRGB.