Richard of york gave battle in vain is a mnemonic to remember what?

Answer:
The colors of the spectrum, in order, are

red
orange
yellow
green
blue
indigo
violet

So the quoted expression appears to be one of several mnemonics for the ROYGBIV sequence.
First answer by ID1506791434. Last edit by SapphireMoon. Contributor trust: 905 [recommend contributor recommended]. Question popularity: 2 [recommend question].