Since I discovered that red, blue and yellow are not the primary colors, I wanted to learn more about color theory.

One Art Term Glossary uses this definition for a primary color:

A fundamental color that cannot be separated into any other colors. When primaries are mixed, they can produce all the remaining colors.

But why are there three primary colors and not two or four?

It turns out that the answer is purely biological. Without getting into detail, I’ll sum up what I learned from Wikipedia:

• Different wavelengths of light produce different colors.
• Humans can only see a limited spectrum of light, called the visible spectrum.
• There are three types of cone cells in the human eye, which reduce light to three signals.
• There are actually two sets of primary colors, one for the additive color system (used with direct light) and one for the subtractive color system (used with pigments).
• From the page on Additive Color, I found this interesting: “There is a vast difference between Yellow light, with a wavelength of approximately 580nm, and a mixture of red and green light. However, both stimulate our eyes in a similar manner, so we do not detect the difference.”

So, from what I gather, there are three primary colors because we have three different types of cone cells in our eyes. Mixtures of different kinds of light trick our eyes into seeing a certain color, which is why we can reproduce all the colors of the visible spectrum by mixing the three primary colors.

But then I found a long (very long) article titled Do “Primary” Colors Exist?, which concludes that primary colors are practical for many purposes, but they’re still arbitrary. Can the primary colors really create every color?