Assuming you're not trolling us by posting something from the Yellowstone region, then my guess is a Colorado river cutt with a metric ton of salt grains.
The "east side=greenback, west side=colorado" isn't quite true anymore. Recent genetics work on Colorado's native cutthroat has formed a case that a variety of state and federal agencies were stocking a fish on the eastern slope that was probably a greenback/colorado river cutthroat hybrid. There's also evidence that the Arkansas basin was home to a genetically distinct form of cutthroat from the South Platte basin, though last I heard that one's still up for debate? I dunno. Anyways, if you want to learn more about Greenbacks, including how to identify them and where to catch them, you should really check out the following sources:
A plain-English report (with all known recovery locations to date) can be found from Trout Unlimited and Western Native Trout Initiative: [log in for link]
If you prefer your information in the form of a story, you can find a good write-up in HCN: [log in for link]
If you want more nitty-gritty details about Greenback Cutthroat trout morphology, genetics, and the current recovery effort, check out the CPW's Greenback Cutthroat page: [log in for link]
TLDR: CO's native cutthroat are a mess on the eastern slope. To my understanding, the only thing that's clear is that locations with Bear Creek (aka "true") Greenbacks are well documented (see the TU/WNTI report above). If OP was fishing at one of the recovery locations, then it's a greenback. If not, it's...something else.