E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Knights of the Rainbow.
Flunkeys; so called from their gorgeous liveries.
The servants who attended them contradicted the inferences to be drawn from the garb of their masters; and, according to the custom of the knights of the rainbow, gave many hints that they were not people to serve any but men of first-rate consequence.Sir W. Scott: Redgauntlet, chap. 20