Harford's Sulphur, Colias harfordi
Harford's Sulphur butterfly is a little smaller than our Dogface, a little calmer than the Alfalfa or the Dogface. Quite yellow and it likes buckwheats.
I've not noticed Harford's butterfly before, but it's range is from about Monterey south into Baja. Larvae live on Astraglas and other members of the Pea Family.
Also known as Colius alexandra harfordii.