Growing up, I was taught that rainbows have 6 colors. Red, orange, yellow, green, blue, and purple. It makes sense as it goes from primary color (red) to secondary color (orange) back to primary (yellow) back to secondary (green), to primary (blue) and finally ending on secondary (purple). Indigo is a blend of blue and purple, but purple is a blend of red and blue…..
In: Physics
There is some evidence that ability to discriminate different colors depends on the number of words for different languages–different cultures depict them slightly differently and teach numbers of colors differently. In American classrooms they’re typically taught as seven colors (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
That doesn’t really answer your question, though. The electromagnetic spectrum and the color wheel are separate things. If you mix red and blue paint you get purple, but that is not a simple physical relationship.
Colors as we perceive them are a function of the wavelength of the photons (ranging from around 400-800nm roughly). The colors we perceive essentially band these wavelengths together into a color we describe with a word (ranging from violet at the lower wavelengths to red at the higher ones in the opposite order I mentioned earlier). Primary colors are arbitrarily selected–Red, Blue, and Yellow are one set of primary colors, but so are Cyan, Magenta, and Yellow.
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