These butterflies are easy to confuse with orange sulphurs, but generally, these are yellow without any orange. Both species are multiple brooded, and both can use deerweed as their larval food plant. But these tend to really prefer species of Astragalus - rattlepod (such as douglasii). Orange sulphurs may use rattlepod at times, but are best known for their use of alfalfa, and can often be found swarming alfalfa fields. They like clovers as well. So while there is overlap in food plants, these species do have their preferences: Harford's tend to be around rattlepod, while orange sulphurs love alfalfa.
Colias harfordii at Grizzly Flat in the San Gabriel Mountains, August 6, 2011. This one was roosting in the shade in the morning.
Out in the desert, a Harford's sulphur taking a break. This was at Cactus Spring Trail on May 21st, 2016.
A Harford's sulphur about to leave this flower - these never perch with wings open. They can be difficult to distinguish in the field from certain yellow forms of the orange sulphur with which they often fly. But Harford's is yellow without orange on the dorsal side, and the black edging on the wing is usually narrower than in orange sulphurs. Little Dalton Canyon, July 8, 2006.
Another Harford's sulphur. July 9, 2022 along Jacoby Canyon Road in the Big Bear area.
I found a female ovipositing deerweed (Acmispon glaber) along Keen Camp Summit Road in the San Jacintos on May 15, 2022. I didn't keep the egg, but here is a shot I got in the field.
This is a pretty typical Acmispon glaber plant from Box Canyon in Anza-Borrego.