I probably ignored C. occidentalis because (1)'s two photos are yellowish rather than greenish. He also says that the HW spot has a dark red border and the FW spot is often faint, neither of which points seem to match mine. But, that information is rather too thin to rely on. The female photo you linked to does look a lot like mine.

They can be a problem to identify, especially when in the context of one photo of one side (easier when looking at a wild population of numerous individuals - up close and personal). Anyway, I couldn't resist just a tad more comment - hopefully useful. If you look at Colias interior photos (or specimens) you'll see that the shape of the wings is usually substantially different, especially the front wings, with the wing proportionately shorter, usually a bit more rounded on the outer margin. While perhaps "pointed", it is to a lesser degree, and not accented as much by the shape of the wing; the overall shape not being quite so narrow and "stretched" toward the point. It's subtle, but easy to see with a series of specimens side by side. The other thing I notice (just a clue, not definitive) is there is no trace of the dark band on the upper side showing through. Usually you can see it if it's there (even on the under side), and usually it is present (though often quite reduced) on female C. interior. It is quite often (usually in many regions) totally missing in female C. alexandra, making white females look almost more like some sort of Cabbage White than a Sulphur. The shape and character of the spot on the lower hind wing (on the front wing for that matter too) tends to be moderately constant in most species, and there is an average "look", but there are always individuals in which it is outside of the "norm". Same for the pink spot rim, wing fringe and tinge at the base of the wings. Too bad that books like Glassberg's can't have 20 photos of every gender of every species, but of course they can't. I can't rule out C. gigantea, but as you said, it shouldn't be there according to the maps, the shape of the wings is usually a bit broader and less "pointed" but not enough different to rule it out. Same comment as to distributioin for C. christina (which it really looks a lot like). C. interior I seriously doubt, but I can't rule it out either. One more contributing complication, is that it seems likely judging from the occasional weird specimens, that all of the species that feed on Legumes can and sometimes do hybridize. I am hoping that these added tid-bits will be helpful for anyone opening this page - and not too "long-winded" :)

Thanks for the additional clarification. I guess I'll relabel the image as (tentatively) alexandra and leave it at Colias. It's too bad that BugGuide itself seems to have few if any clearly identified C. alexandras.

It would be a hard bug to photograph, especially the females. I never caught many when I was a kid, not because they were rare, but because I couldn't catch the wary little speed demons. On a rare occasion one will sit on a flower and ignore you, but not often. Perhaps that is part of why there are so few photos here. C. philodice and C. eurytheme are much easier to approach (usually).

Perhaps if you or somebody else gets more photos from the same area, the identification will eventually become more certain. There are a few Colias fanatics out there who might already be familiar with the same population too, and perhaps we'll hear from them? I'm very curious about this one myself.