I think you might have a male Brown Widow on your hands. Could be female, i'm not sure.
Ramshackle Solid: Male Brown Widow on Our Dish Rack
Cellar spiders have long spindly, string like legs and tiny in comparison bodies, and no visible mandibles.
The fellow you've got there has decently thick legs in comparison to the body, and an oddly shaped / colored abdomen. I looked up whether brown widows can be found in Colorado, and while not common they can be found there.
Another reason I believe it's a male brown widow are the lengths of the legs - the two front and two back are extremely long while the two middle pairs are shorter.
I think the main issue is a lack of reference for size comparison ^.^' About how big was the spider?