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Black with a a yellow stripe down the back. white and brown below. A silvery gray head.
Gardens, Grassy fields, Prairie, Lawns, Ponds (can swim)
Also, I should add, nice photo! Garter snakes are very hard to identify because there are so many different subspecies and color variants. It takes a good photograph that captures an identifying feature to ID some of these guys.
Based on your spotting location and those black spots near the orange dorsal stripe, this looks to me like a subspecies of the Western Terrestrial Garter Snake, the Wandering Garter Snake.
Indeed a Thamnophis sp. but does the Texas subspecies (Thamnophis sirtalis annectens) occur in Colorado?