This odd gull was at Valmont Reservoir complex on 20 January 2007. The photos here were taken by three separate observers with different cameras and scopes.

Below are my thoughts on the possible identities of this bird. These possibilities assume that this is a first-cycle bird (about 7 months of age).
1. Great Black-backed Gull - too small, scapular molt too advanced, gray in scaps too pale.
2. Lesser Black-backed Gull (graellsii subsp) - too big, bill base too pale, gray in scaps too pale
3. Lesser Black-backed Gull (heuglini subsp) - Maybe, correct size. AKA Siberian Gull
4. Caspian Gull (L. cachinnans) - Maybe. Size and plumage similar to the cachinnans or barabensis subspecies (AKA "Steppe Gull" in Olsen and Larsson's gull ID book). Hard to imagine a west Asian gull reaching Colorado.
5. Yellow-legged Gull (michahellis subsp) - Bill base too pale and head too white
6. Herring Gull - Head too white (for all subspecies)
7. Hybrid - Herring Gull x Great Black-backed Gull is a possibility.

Click on photos for larger image.