HCR -
Sounds to me like a drake - haven't had the pleasure of fishing in CO, but around here they come in brown, gray, and green, depending on the stream / river and the time of year, and it is usually during the summer months.
We do get some large PMD's around here, but I don't think I have seen any I would consider a true size 12 dry fly, probably a sparse 14 at the biggest, and they definitely do not get into the gray range of color in this neck of the woods.
Right now we have a Western Green Drake hatch going on on one of my favorite cutthroat streams. Been fishing a size 12 extended body dry fly pattern, which the fish have been much liking, although they have not been particularly selective. Different water, different species.
If you go down to the Fly Tying Forum you will find a thread on the Western Green Drake. Will give you an idea of what has worked up here recently - and hopefully will again tomorrow.
John
The fish are always right.