The only other White that I think would be possible in that area would be Pon*tia proto*dice (Checker*ed White). I've never seen an adult in her garden before, but I found myself considering the possibility after looking at this page showing a larval photo on the website "Arthropods of Orange County". It certainly looked similar to mine! Then, I went and viewed more images on this site as well as others, and they didn't look anything like the first picture.

So, do you think is it possible that there is significant variation in the Pon*tia proto*dice caterpillar, or is it simply a case of the first photo I looked at being misidentified?

I don't know if the caterpillar shown at the first page is the same individual as the pupa to its right, but if you look at the pupa, it is clearly a Pieris rapae about to emerge. Variation in color is always a possibility, but I've never seen a caterpillar of P. protodice that was all green like this. I would need to compare, but if I remember correctly the "hairs" or "bristles", or whatever they are, are a bit different on the two species too. I've seen Pieris napi (or perhaps I should say P. marginalis) caterpillars that look very like P. rapae, but I'd think that species probably wouldn't be in this location?

...I'm pretty sure the photos of the caterpillar and pupa are misidentified. And, since you are correct that P. margin*alis would not be found at this location, I feel confident that this is in all probability a Cabbage White. There are already much higher-quality photos of this caterpillar in the Guide, but I'm going to go ahead and move it to the page because (believe it or not) there don't seem to be ANY records at all for this species in the month of February.