Birds feeding for colour

I have often considered the feeding preferences of certain birds and wondered whether or not this related to the colour of the birds. Recently I saw for the first time, bullfinches feeding upon the seeds of Buddleia. I have previously seen them feeding upon the seeds of Buttercups, could the flower colour be passed on to enhance the feather colour of the birds? It is well known that canaries kept in captivity can be turned to a red feathered variation through the means of colour feeding. Redpolls, siskins and goldfinches all commonly feed upon the sees of alder trees, these are noted for their redness.