If the additive and subtractive colors actually describe how the mixing of colors work, how come treating red, yellow, and blue as primary colors also works? At some point, every one of us has tried mixing these colors, either as food colors, as paints, or some other way, and we were able to confirm that red and yellow make orange, yellow and blue make green, and blue and red make purple. How does that work if the primary additive colors are red, blue, and green and the primary subtractive colors are yellow, magenta, and cyan?
First of all, note how red, yellow, and blue are treated as subtractive colors: orange, green, and yellow are obtained by mixing pigments, be it paint or food coloring. Thus, let us apply our knowledge of subtractive colors to see if we can figure out what is going on.
What happens when we mix red pigment with yellow pigment? Red pigment reflects red light and absorbs green and blue light. Yellow pigment reflects red and green light and absorbs blue. Thus, if they are mixed together, red will be reflected just fine, since neither pigment absorbs it. However, the green reflected by the yellow pigment should be absorbed by the red pigment. We must consider that most pigments are not one hundred percent effective in practice. That is, in theory red pigment should absorb all green and blue light, but in reality, it only absorbs some of the green and blue light and reflects a little bit. Thus, the green reflected by the yellow pigment is not absorbed by the red pigment completely and a little bit of green gets reflected. What color is made by red light with a little bit of green? Orange.
Now let us consider yellow pigment with blue. Yellow pigment reflects red and green light and absorbs blue while blue pigment reflects blue light and absorbs red and green. These are complimentary colors and should produce black. Why then is green produced? I think the reason is that many blue pigments actually reflect a mixture of blue and green. Consider how cyan, which as an additive color is a mixture of equal parts blue and green, looks similar to blue. While it has a hint of green, it can almost pass as a light blue. What if a pigment were to reflect a lot of blue and a little green? It would lie somewhere between cyan and blue in terms of color, or it would simply look like a shade of blue. Now, let us consider such a shade of blue pigment mixed with yellow. The “blue” reflects blue and green and absorbs red and the yellow reflects green and red and absorbs blue. Blue gets absorbed by the yellow and red gets absorbed by the “blue” leaving green.
Finally, what about mixing blue and red pigments? Blue pigment reflects blue light and absorbs red and green while red pigment reflects red light and absorbs blue and green. Thus, blue light is absorbed by the red pigment and red is absorbed by the blue pigment and we are left with black. Actually, rather than being black it will probably be more of a dark, muddy blue or muddy red color, again because absorption will be less than one hundred percent, but it will not produce purple. However, just like cyan is close to blue, notice that magenta is close to red. What if we mixed blue with a “red” that was actually close to magenta? The blue reflects blue and absorbs red and green while the “red” reflects red and blue and absorbs green. Blue is reflected by both pigments but red will be absorbed by blue. If the absorption of red by the blue blue pigment is incomplete, then a little bit of red from the “red” will be reflected. What is blue light mixed with a little bit of red? Purple.
In short, the reason red, yellow, and blue as primary colors and orange, green, and purple as secondary colors works is because the pigments of these colors are not true pigments (not true blue, and true red) and absorption of complimentary colors is less than perfect. It work because the real world is messy while the primary and secondary additive and subtractive colors reflect more ideal circumstances.
Having read all of this, one may wonder why the primary and secondary additive colors do not match the rainbow. After all, red, yellow, and blue are spaced so nicely on a rainbow, it just makes sense that they are primary colors. If red, blue, and green are the primary colors, why is red separated from green, the closest primary to red, by orange and yellow and why are green and blue, two primary colors, adjacent to each other? The answer is that a rainbow is created by the refraction of light in raindrops. Refraction separates light by wavelength. Red has the longest waves while purple (violet) has the shortest waves and as we progress from red to orange to yellow to green to blue to violet, the wavelengths become progressively shorter.
Why do the primary additive colors not reflect the wavelengths of light? Because they are determined by a completely unrelated mechanism. Red, blue, and green are the primary additive colors because those are the colors of light we can detect with our eyes. Our eyes contain two types of light sensitive cells: rods and cones. Rods are simply sensitive to light: it does not matter what is the color of the light, they are activated by any light at all. Rods are responsible for our ability to see black and white. The cones, however, are sensitive to color. There are three types of cones in our eye: one that is sensitive to red light, one that is sensitive to blue light, and one that is sensitive to green light. Of course, we see more than just red, blue, and green, and that is because our brain interprets relative amounts of these primary additive colors as other colors, such as the secondary additive colors. In other words, while the mixing of primary and secondary additive and subtractive colors relfect the properties of light, the actual identification of the primary additive colors comes from the cells within our eyes. That is why the spectrum of the rainbow does not reflect the primary additive colors.