This bird was a mile away (back, center). Observers in the field all felt that sides were pale enough to be the first documented Atlantic Brant (Branta bernicla hrota) for Colorado. Black Brant turns up most years. This bird was found by yours truly, and accepted by the CBRC as a Brant. I do not know if the committee ruled on subspecies. Photographer Bob Smith was present and took this long-distance shot.

UPDATE: The weak contrast between black chest on pale belly probably indicates Gray-bellied, rather than Atlantic.