Children are often taught that Red, Yellow, and Blue are “the primary colors”, with Orange, Green, and Purple “the secondary colors.” This matches neither additive (RGB) nor subtractive (CMYK) color models.
- Is/was there a prominent color system which used Red, Yellow, and Blue as primary colors?
- If not, why are Red, Yellow, and Blue often taught as primary colors?
Sorry, but you’ve made some assumptions that aren’t quite right. Anyway I’m going to try to keep this simple and try to avoid jargon. It’s not an easy subject for the uninitiated, and there’s a lot of science behind it too which I won’t really delve into here.
“Additive colour” applies to mixing colours of emitted light. There’s nothing to stop you from mixing any colours of light. It’s not only limited to RGB. The RGB system is used because it closely resembles the way we humans see and perceive colour using the red, green and blue cones in our retinas.
“Subtractive colour” applies to all colours created by using physical mixes of pigments in media such as paint/ink/crayons/pastels etc, pigments which absorb certain frequencies of light, and reflect others. There’s nothing to stop you from mixing any pigments to get another colour. It’s not just limited to CMYK.
RYB isn’t only taught to children, and it’s taught for a reason. The idea of using red, yellow and blue as primary colours was basically invented by painters/artists. Systematic theories about it date back at least four centuries (there’s a wikipedia article here if you’re interested), and no doubt even the ancients knew they could mix paints to get different colours although their understanding of it might have been somewhat limited. That system is still used by artists today. Of course artists are not limited to mixing just red, yellow and blue colours, but have a whole slew of pigments at their disposal.
Since all physical mixes of paints/inks work using subtractive colour, RYB is also a Subtractive colour system, just a different one from CMYK which was specifically developed for full colour printing. It’s not even used for all colour printing. For example Pantone uses a basic set of 18 base colours for creating spot colour ink mixes.
The RYB system eventually evolved into CMY, because basically when people discovered how to print images in colour by overlaying different coloured halftone screens, they realised that RYB wasn’t the best system for reproducing colour photographic images in print. Also, inks are a bit different from paints in that they are transparent, and the colour of the paper always shines through the ink. Paints tend to be rather opaque by comparison. In time CMYK became the standard used for full colour printing today. Black was added because dark areas in CMY only tended to look a bit muddy/brownish rather than black. Black isn’t really a primary colour. Technically speaking it’s not even a colour, but a lack of colour.
Note: Although I use the term “mixing” rather freely above, it’s also quite important to note that CMYK/process printing involves no real mixing of inks as such. Rather, colour “mixes” are achieved by overprinting four halftone screens on top of each other, one for each colour – essentially creating the illusion of different colours being “mixed”. When viewed under a magnifying glass, dots of solid CMYK ink can easily be seen in a full colour print. By comparison, paints or spot colour inks are physically mixed together before they are applied.