As always, it's hard to discuss a bird like this, which I observed in the
field for an appreciable amount of time, with someone viewing two distant,
clearly imperfect photos. With the myriad problems presented by photos and
lighting itself, you really must be careful in trusting mantle color
appearance. That being said, the mantle color seems fine on my monitor,
showing the dark slaty back with very little blue tone-- but again your
experience may vary depending on your monitor.

Gene Hunn mentioned Vega Gull. This is the taxon of "Herring Gull" found in
Siberia and fairly common (locally) in western Alaska. They are darker than
our (smithsonianus) Herring Gulls but they're not by any means slaty. Do a
google search for Japanese Gull Site if you're interested in seeing an
amazing collection of photos. If you're familiar with Slaty-backed, you
shouldn't easily confuse a pure one with Vega Gull. Importantly, Vegas still
look like a Herring Gull type bird. Slaty-backed is a very distinctive bird
in shape, although highly variable in size. They don't have the same
structure as a Herring Gull. The structure was what jumped out on this bird
and made it possible to identify as a Slaty-backed in a matter of seconds.
The bill is indeed quite slim, and the head very rounded, suggesting a
female, but the bird still has the short-legged, long-necked, small-headed,
pot-bellied look of a Slaty-backed Gull.

Mantle variability in Slaty-backed is an interesting topic. There have been
conflicting statements published in the literature, with some allowing a
great deal of variation in mantle tone, while other authors have said paler
birds are the result of hybridization with Glaucous-winged Gull. The extent
of that hybrid zone is controversial and not well known to me. It's worth
noting that most of the Slaty-backed Gulls that have reached the east coast
recently appear to be fairly pale-mantled, which has raised the possibility
of some of them being hybrids. As far as I know, pretty much all the
Slaty-backed that have been found on the west coast seem to be very typical
in mantle shade, though I've been keeping my eye out for the same variation.

Incidentally, and strictly to confuse matters, there was an adult Herring
Gull hanging out with the Slaty-backed that showed virtually a full suite of
Vega Gull characters, and was possibly outside the range of smithsonianus
mantle color. The problem is that nobody really knows what IS the range of
smithsonianus mantle color, at least to my knowledge. The other "problem"
was that I was quite distracted by this brilliant Slaty-backed Gull!

Cheers
Charlie Wright
Bonney Lake, Washington