Two of the most helpful references for Slaty-backed ID are Moores (2005) and Zimmer (2000):
Mantle color variation in Slaty-backed has been debated since Gustafson and Peterjohn's Birding paper in 1994. The most popular notion is that a paler mantle is attained through hybridization with Glaucous-winged. This is mostly based on the fact that paler-mantled birds are scarce among the "pure" Slaty-backed Gulls in Japan. Birds with paler mantles are still often called Slaty-backed in CA, including one of the current Richmond birds. I feel this is valid because the presence of Glaucous-winged genes does not preclude the bird from fundamentally being a Slaty-backed. Gene flow is an adaptive strategy for these birds. If we only counted "pure" birds, then the Great Black-backed Gull is most likely extirpated in the New World (most have Herring haplotypes). Alternatively, we could just consider them all Aralo-Caspian Gulls, as the world was quite different the last time large white-headed gulls acted like biological species!
Also note that both Herring and Slaty-backed show various shades of gray and blackish in the first alternate scapulars, which may or may not reflect the adult color.