Dark adult
Kamtchatka Gull? Larus canus kamtschatschensis?
 Abashiri Hokkaido Japan, December 14th 2000.  Photos by Yoshiaki Watanabe 
A dark adult in center, with 2 normal Kamtchatka Gulls.
Such dark adult 'Common Gulls' (often similar in mantle tones to Black-tailed Gulls) that are found in small number in Japan were supposed to be possible race heinei, according to Grant "Gulls" 1986. But the idea seems getting doubtful, according to recent literature.

Another similar example at Tamagawa-river,
Kawasaki kanagawa Japan, on January 5th 2000
 Photo by Osao Ujihara