Black-capped Chickadee, Beaverton, Oregon, 7 December 2011 by Greg Gillson.
Sibley, in his account of Black-capped Chickadee, has a box explaining the ID pointers that will separate Black-capped from Carolina Chickadee. This is an identification challenge we do not really have to worry about here in the Pacific Northwest.
Sibley states that the Black-capped Chickadee "has a greenish back and buffy flanks."
Hmm... the back of Black-capped Chickadee has always appeared rather medium gray to me. But now that you mention it... does the back have a greenish cast in the photo above? Perhaps.
Gabrielson and Jewettt in their 1940 book Birds of Oregon, use Florence Merriam Bailey's textual descriptions from her 1921 Handbook of Birds of the Western United States. That description states: "back dark gray, tinged with olive brown."
Please see a previous post on Black-capped Chickadees: (In the backyard... Black-capped Chickadee)