A friend of mine told just called and I sent him to this page. He is familiar to some extent with carotenoids in birds and he said that red birds synthesize from dietary carotenoids either deep red pigments (astaxanthin, canthaxanthin, etc) which is typical for the Cardinal, while in others, such as the house finch mentioned above, intermediate red pigments are made, such as 3-hydroxy-echinenone and echinenone (no chemistry needed, these latter pigments are less red then deep red pigments). Also there is a mutant cardinal with only yellow feathers in a Louisisanna museum (on the web) and so cardinals can produce yellow feathers as well.
Perhaps metabolically this orange cardinal is only producing small amounts of deep red, or perhaps more intermediate red (a mutation or other anomoly) and perhaps a little yellow as well (which would not be normal). Since he is the only cardinal this color I have seen here I assume it is not diet related. So, probably, as my friend suggested, a metabolic anomoly (should trap him and get a feather for analysis, eh?)