Although there is a snake native to Colorado called the blackhead snake (genus: Tantilla), I'm fairly certain yours is not it. The name seems to match the snake perfectly, but yours is a juvenile, and has rough scales. Aside from having a black head, it doesn't match.
I'm with Rachel, I believe its a garter snake, and so far I think its a western terrestrial garter (
Thamnophis elegans). There's also the possibility that its a plains garter (
Thamnophis radix), but I'm more confident in the elegans. Could you get any side shots? Particularily of the head.
Garter snakes are extremely variable in color and pattern, but an elegans with your pattern is very possible:
http://www.bluechameleon.org/Field%2...ans,%20juv.jpg
however, the same can be said of radix:
http://www.ryanphotographic.com/Plai...is%20radix.jpg
The blackneck garter has a name that seems to match the snake, but a juvenile of that species has far more color and patterning to it typically:
http://www.egsa.de/galerie/images/005.jpg