A question about a snake we found in the yard
I am confident your snake is a western terrestrial garter snake. They are common in Colorado. Garter snakes often have a stripe down their back but it isn't always prominent. A western terrestrial garter snake isn't poisonous.
Snakes often roam while looking for food. It may be passing through on it's search.
The Colorado Herpetology Society has pictures of snakes on their web site.