This larger, paler form of the California Gull (which breeds in Alberta) occasionally occurs in Colorado, but has not been documented to my knowledge. This presumed albertaensis was at the Larimer County landfill on 6 Dec 2014.

Unfortunately the manual digiscoping process has distorted the brightness compared to real life (whites are whiter and darks are darker than reality). However, in these shots you can see typical Cal Gull characters such as yellowish legs, dark eye, and black-red spot on bill. Note that the size is larger than nearby californicus gulls, causing this bird to tower over the many Ring-billed Gulls in the shot. The shade of gray of the mantle is intermediate between Ring-billed Gull and californicus California Gull.