Yellow, symbolic of treason. In the ancient tapestries Cain and Judas are represented with yellow beards. (See Yellow .)
“He hath but a little wee face, with a little yellow heard, a Cain-coloured beard.” —Shakespeare: Merry Wives of Windsor, i. 4.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894