I was tied up with work, but was going to suggest A.krausei as a possibility. While they are not found much east of the Rockies, they do occur relatively close to Missoula, and very small animals I've seen have only been faintly marked at first. I've seen A.macrodactylum change shades from distinctly striped to almost uniformly dark, but haven't actually seen that happen in A.krausei. Again however, absolutely not a newt, but an ambystomid. Montana is also home to Plethodon idahoensis, which this certainly is not, and Dicamptodon aterrimus, which has a very limited known range in Montana [but is very Ambystoma-like].