1st winter to summer Slaty-backed Gull Larus schistisagus
Sakawagawa river, Odawara, Kanagawa, Japan, April 2nd 2001.
These following 2 images show typical tints of first winter/summer Slaty-backed Gull.
Upper parts from lower breast are distinctly whitish, against brown belly.
This often can be good key to identification.
Smithsonianus also often shows whitish head and neck.
But, I feel it usually starts from upper neck in smithsonianus
though it may depend on seasons too.

First summer birds often acquire very whitish appearance, and sometimes 
are confused with Glaucous-winged or Glaucous Gull.
But worn Slaty-backed usually gives us brownish, patched impression.