Mnemonic Device:

• ROY G. BIV

Variations:

• Read Out Your Good Book In Verse
• Reports Of Young Girls Banging Inside Vatican
• Richard Of York Gave Battle In Vain
• Richard Of York Got Buggered In Venice

Explanation: Mnemonic devices for the order & colors of the rainbow (Red, Orange, Yellow, Green, Blue, Indigo, Violet).

Or reverse/inside out:

• VIBGYOR

Light is just a part of the spectrum of EM-waves: see our mnemonic on all EM-waves

More information on Wikipedia

Mnemonic (double) rainbow