The garter snake family, which consists of many different species all across the continent, is pretty universal in having a stripe down the middle of the back. The stripe can be red, orange, yellow... but every garter snake species I can think of off hand has it.
The great plains rat snake
ranges in Colorado. 12 inches would be about the size of a baby, they grow fast and can get upwards of 4 foot. They tend to prefer grassland type habitat at the edge of light forest, and aren't found at altitude.
Also in Colorado you have the coachwhip, night snake, and glossy snake which all could fit your description easily, and being that it hangs around a pond, it could even be one of the water snake species. If you had a picture, or more details, I could narrow it down better.