I am sure you have noticed that the amount of red on adult male house finches is highly variable. Throughout most of the year, what a bird eats affects amount of red it displays. Specifically, the birds that consume foods containing large amounts of carotenoid pigments (e.g. fruits and berries) have plumages that are redder than those birds that eat foods containing little, if any, carotenoid.
In addition, when a male house finch is developing new feathers, the fresh feathers are brighter tha those they replace.