But really, before you knew the mnemonic if someone asked, "What colors do you see?" what would you have said?
I don't know if anyone has ever tested young kids before they are exposed to science classes, but it seems it used to be common to describe four colors, Red, Yellow, Green, and Blue... So why do we see (or why are we told to see) seven?
Turns out, it was old Isaac Newton, and his dedication to the ancient Greek ideas. Here is a clip from a blog by Patricia Fara, a senior lecturer at Cambridge, and author of "Science: A Four Thousand Year History" at the Nature web site.
Consider Isaac Newton. He believed so firmly in the Greek idea of a harmonic universe that he divided the rainbow into seven colours to correspond with the musical scale. Before then, although opinions varied, artists mostly showed rainbows with four colours. It is, of course, impossible to make any objective decision about the correct number, because the spectrum of visible light varies continuously: there is no sharp cut-off between bands of different colours, so how you think about a rainbow affects how you see it. Be honest - can you tell the difference between blue, indigo and violet?
I found this on Wikipedia: "In Classical Antiquity, Aristotle had claimed there was a fundamental scale of seven basic colors. In the Renaissance, several artists tried to establish a new sequence of up to seven primary colors from which all other colors could be mixed. In line with this artistic tradition, Newton divided his color circle, which he constructed to explain additive color mixing, into seven colors. His color sequence with the unusual color indigo is still kept alive today by the Roy G. Biv mnemonic. Originally he used only five colors, but later he added orange and indigo, in order to match the number of musical notes in the major scale."
So I can't find out who first came up with the mnemonic, but I did find a song about it...lyrics are here. And Aaron Wagner sent me a link to a video of a different ROY G BIV song by "They Might Be Giants" that his four-year old highly endorses. Thanks to Aaron and his child for the tip.