To look yellow; to be jealous. I happened to call on Mr. Green, who was out: on coming home, and finding me with his wife, he began to look confounded blue, and was, I thought, a little yellow.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Yea and Nay Man * Yellow BellyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes