Why is color not mixable?

Mixing colors

Enlarge image
How does mixing colors work?
How does mixing colors work?

In the additive color mixing three light sources with the primary colors red, green and blue are sufficient to generate white light if their rays are mixed in the same ratio, i.e. with the same intensity. The result of additive mixing can be understood using an animation (picture on the left): What is the result of adding the red light spot to the green or the blue or the green and blue spot? For the subtractive color mixing we use a virtual brush and “paint” with the primary colors yellow, magenta (purple) and cyan (blue) on a white sheet of paper. Depending on which colors are mixed with each other, there is a different mixed color.

When comparing the two mixing methods, two basic properties stand out: With additive mixing, the light spots in all three colors produce a pure white where they meet in the middle. With subtractive mixing, however, a black is created there. The absence of any color means pure black in additive mixing, but white in subtractive mixing. So additive color mixing and subtractive color mixing have opposing properties. But why is it like that? And when does which of the two mixed systems apply? The animations give an indication of an answer to the second question: Additive color mixing has to do with light, subtractive mixing, on the other hand, has to do with painting (or printing).

Anyone who has systematically worked through the mixing of colors in the paint box will make another interesting discovery: some colors cannot be produced by mixing other colors. They are red, yellow, blue; there is also the "achromatic" color white. The also “achromatic” black, on the other hand, is achieved by mixing red, yellow and blue in equal proportions. However, it is a rather brownish-dirty black. Incidentally, with watercolors, you cannot paint white at all (with watercolors) or at least not particularly well (with gouache paints) over other colors. This is only possible with pastes for oil paints or acrylic paints, if only because they can form thick layers.

So red, yellow and blue play a special role in the color box. That is why they are called basic colors in art or - in technical-scientific fields - Primary colors. But why are there primary colors at all? To understand this, one has to look at the function of our sense of sight. And when you do that, you end up with an even more fundamental question: What is color anyway?