“Richard of York gave battle in vain,” for the colors of the rainbow

MNEMONIC

Think of a memory trick that helps you recall the sequence from red to violet—it’s not an acronym but a phrase designed to stick.

The phrase "Richard of York gave battle in vain" is a well-known mnemonic device that helps people remember the colors of the rainbow in order (red, orange, yellow, green, blue, indigo, violet). The clue gives an example of a mnemonic, which is a memory aid, and the answer is simply the word for that type of tool. The wordplay is straightforward—it’s not a riddle but a direct reference to what the phrase represents.