Update: Jonathan Pelham has confirmed the ID as Colias canadensis.
These are two of the four Sulphurs Colias seen flying patrol routes along the low-point on the side of the Glenn Highway at Sheep Mountain Lodge on June 01, 2008. The gravel road bank at this point had numerous dandelions flowering, and each Colias occasionally stopped to feed on them - but otherwise they flew very low to the ground along the border of the low dense scrub with numerous scattered spruces that covered the lower part of the valley, with the treeline occuring c. 100 feet higher up the valley slope. None were seen above the height of the road surface, and they seemed to arrive at the road edge from low within the scrub belt. The habitat and date seem to favor C. canadensis, as does the somewhat narrow black wing borders, rather limited green tinge to the hindwing, and rather trangular HW white spot that points outwards... yet the limited data I can find on the range of this taxon indicates that it has not been documented this far west: