My first thought was an old western movie or radio show where the bully would say, “what’s the matter, are ya yellow?” Then finding these definitions I find it is not so amusing any longer.
Definitions of yellow from WordNet1yellow (adj.)typical of tabloids;
Synonyms: scandalmongering / sensationalisticyellow (adj.)of the color intermediate between green and orange in the color spectrum; of something resembling the color of an egg yolk; Synonyms: yellowish / xanthousyellow (adj.)easily frightened; Synonyms: chicken / chickenhearted / lily-livered / white-livered / yellow-belliedyellow (adj.)changed to a yellowish color by age;
Synonyms: yellowedyellow (adj.)cowardly or treacherous; “the little yellow stain of treason”-M.W.Straight;
too yellow to stand and fight
yellow (adj.)affected by jaundice which causes yellowing of skin etc; Synonyms: jaundiced / icteric2yellow (v.)turn yellow;
The pages of the book began to yellow
3yellow (n.)yellow color or pigment; the chromatic color resembling the hue of sunflowers or ripe lemons; Synonyms: yellownessFrom wordnet.princeton.edu