I've been out of town for the last 10 days; so, sorry for replying to this inquiry so belatedly. Although the variability in yellow to reddish coloration in adult male (and to a much lesser extent in adult female) House Finches MAY reflect differences in their diet (see references mainly by Geoff Hill from 1990 to present), there also is considerable data to show that this variability is likely caused by differences in the health of males at the time they molt: less healthy males grow yellow to rangish plumage whereas the haelthiest males grow more reddish plumage. This was first documented in the reference below and has been further supported by more recent work by Geoff Hill.

Thompson, C.W., N. Hillgarth, M. Leu, and H.E. McClure. 1997. High parasite load in house finches (Carpodacus mexicanus) is correlated with reduced expression of a sexually selected trait. American Naturalist 149:270-294.

Best regards,

Chris Thompson
Research Scientist
WA. Dept. Fish and Wildlife