Pekin with yellowed comb??

One of our girls literally went off colour for a couple of days and was pretty miserable. Her comb near the skull went mustard yellow   She has since bounced back and we guessed she had eaten somthing odd - all of them have recently feasted on the pyracantha berries.
Any ideas on what could have caused it ?

Did not intend to feed them  - but as the other wild birds eventually strip them off I thought the chickens had just got ahead of the game - nothing left on the bushes up to 2 feet.