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
Fundamentally, what we call the colors of the rainbow are somewhat arbitrary. The rainbow contains a continuum of colors, so there’s theoretically infinitely many of them. We just like to group these into finite categories because our brains handle that better than infinite continuums.
Isaac Newton, in particular, categorized the colors of the rainbow into seven groups: red, orange, yellow, green, blue, indigo, purple. He did this because he thought seven was a magic number (seven days in the week, seven planets in the sky, seven notes in a major scale, etc.) And he was reaching to make this work, including the then-obscure colors of orange and indigo to bring the accepted five up to seven. And this Newtonian naming system has stuck around in public conscience despite not being that useful for color theory, which is where we get the idea of primary and secondary colors.
Given what we know now about human color perception, it would probably make more sense to use a six color system: red, yellow, green, cyan, blue, magenta. The three colors of light our individual cone cells are most sensitive to, and their opposites. Which ones are primary or secondary depends on whether we’re printing with ink or rendering with light.
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