A mnemonic is a device some people like to use in order to help them to remeber obscure things.

Red, Orange, Yellow, Green, Blue, Indigo, Violet (order of colors of the rainbow from long wavelength to short wavelength)

Richard Of York Gave Battle In Vain

Ring Out Your Great Bells In Victory

Read Out Your Good Book In Verse (referring to the Bible)

Ring Out Your Granny’s Boots In Vinegar

A bit of additional trivia is that Isaac Newton added the Indigo color to the list of names of the colors of the rainbow because he thought there ought to be seven colors, not just six. Consider: there are seven days in a week.

Here is a more quantitative approach to the colors of the rainbow.
Wavelengths are given in nanometers (nm). One nm is 1 × 10^-9 m.

Color	Representative Wavelength (nm)	Wavelength Region (nm)
Violet	420	400 - 440
Blue	455	440 - 470
Blue-green	480	470 - 490
Green	525	490 - 560
Yellow-green	565	560 - 570
Yellow	580	570 - 585
Orange	620	585 - 630
Red	660	630 - 700