I think the color indigo was just made up so that people could pronounce the "rainbow" mnemonic "Roy G. Biv" (otherwise Roy's last name would have been just Bv). (red, orange, yellow, green, blue, [indigo,] violet)
Doesn't everyone learn the spaces and lines for the musical notes on the G Clef with "FACE" and "Every Good Boy Does Fine"? The F Bass Clef was "All Cows Eat Grass" "Good Boys Do Fine Always." (There are even some "gay" variations on the last one.)
I hear that medical students have dozens of these that people have worked out over the years.
My absolute favorite mnemonic was one I learned from Harry Lorraine's (and Jerry Lucas') memory book, that replaced numbers with consonants, by which I still know many of my friends phone numbers, pi to 20 places, and which allowed a pioneer buddy and me to work out an amazing card trick where we flipped the card too fast for anyone to see what it was, but we could still identify the card because we'd say things like: Ready? ....flip.... OK, here comes the next one....flip... You'll never get this....flip... All, right this one will even be faster than the last one...flip...
(Of course those were the 4 of diamonds, the 7 of clubs, ace of spades, and 5 of hearts, respectively)
Then there's French verb tricks like: DR & MRS VAN DER TRAMPP which represents all the major French verbs that need the helping verb être rather than avoir. ( devenir, revenir, mo urir, retourner, sortir, venir, aller, na ître, descendre, entrer, rentrer, tomber, rester, arriver, monter, partir, passer)
And lots more I can't remember right now.