I think this may be a Mexican sulphur or Mexican Yellow, but I can't seem to find a good image to compare. The closest thing I could find was at http://www.butterfliesandmoths.org/species?l=1449 with a poorer image than mine but with a good description. My butterfly was nectaring on some red lilies, and I can see what looks like the outline of a dog's face on the upper wing from the underside as well as the tail-like shape of the hindwing.