Light is a wave of energy (or a particle, but for today it’s just a wave OK?) and, like a vibrating guitar string, light waves wiggle at certain frequencies. Some of those frequencies we detect with our eyes, and the frequency determines its color.
Colorimetry is the science and technology used to quantify and describe physically the human color perception. Color sensation is studied through different parameters (usually triads of them) like intensity (amount of brightness), hue (color wavelength) and saturation (purity of the color) that form the HSI system.
Trichromy (the three color process) is the colour theory by which any colour can be reproduced solely combining the three primary colours (red, green and blue) and it relies on human trichromacy of the eye cones. There are, at least, 2 different ways of mixing and representing all colors from three: substractive and additive.
Subtractive Mix (CMY color wheel)
It is due to the absorption of color in inks or paints, which are subtractive and hence the name. A material that absorbs all wavelengths but the 510nm one, will be seen as green, and so on. As it seems on the mixing above, in subtraction mixes the primary colors are cyan, magenta and yellow, whereas the secondary colors are red, green and blue.
As a sidenote, I would like to point out that what in school is taught by blue, red and yellow being the “universal” primary colors is a simplistic representation of the subtractive mix. The thing is that, because perception works in misterious ways, it is quite a good aproximation for paint colors, so painters use a simple RYB color wheel based on this where yellow and “purple” are oppposites and everybody is fine with this.
Additive Mix (RGB color wheel)
Light, on the other hand is additive, meaning if you blast red, green and blue you get white. They are, therefore, the primary colors in additive mixing and cyan, magenta and yellow are the secondary colors.
Its types can be:
- Simultaneous in time and space,
- Alternating in time,
- Spatial yuxtaposition.
There’s nothing in the linear physical light spectrum to indicate that any color is “the opposite” of any other. And the color wheels aren’t much help either; trying to match the “opposites” on the painter’s wheel yields an unsatisfying asymmetry where two of the primaries are opposite, and the third is opposite from a secondary.
But opposites are real. In the early 1800s Goethe noticed that red/green and blue/yellow were never perceived together, in the sense that no color could be described as a combination of those pairs and that by mixing them together you obtain a neutral grey. But wait a minute, what about green? While it is obvious that blue and yellow combined produce green, and green and red neutralise each other, that is not totally true when it comes to light colors.
While yellow/purple and blue/orange are opposites on a painter’s wheel, or red/green seem pretty different from each other, or we can’t get our head around why oh why yellow and blue aren’t producing green, the fact is that there’s something else to this: phisiology and the ridiculous complexity of human beings. Our eye cones and brain stimulus work pretty much more like light than like ink, therefore we must apply the additive notion of color like you see above. As we are dealing with digital imaging, the RGB color wheel is a much better approach than the CMY wheel, and certainly better than the painter’s RYB color wheel.
Have a look now at the digital way of representing color.