The colour of magic.
“Red is the colour of magic in every country, and has been so from
the very earliest times. The caps of fairies and musicians are
well-nigh always red.” —
applied to gold. Hence a gold watch is a “red kettle.”
Thou shewst an honest nature; weepst for thy master; There's a red rogue to buy the handkerchief.
Beaumont and Fletcher: Mad Lover, v. 4.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894