Actually, almost everyone on the west coast seems to think we're ALL mountains (I live a couple of miles from the Bitterroot Range and can see about 6 peaks from where I sit). In truth, we are the 6th windiest state last I read, probably because everything east of the Sapphire Mountains (which I can also see, but from my balcony) is in the Great Plains biome. Two-thirds of Montana is Great Plains!
As for the rocks above, most of the reds are likely shale (also known as fossilized silt layers) the greens could easily be aventurine quartz and various jasper blends, and the whites are likely granite, milky quartz, or stones I'm not great at identifying. And there are a couple of similar lakes OUTSIDE park boundaries
. Flathead Lake, at least the northwestern quarter, also has a wild mix of river-cobble stones.
Heck, even the Bitterroot River, which is a ways from Glacier, spits up spectacular piles of color--I've found shiny-charcoal-colored biotite mica (about the size of a chimney brick), green aventurine that shades to blue on one side, and fossilized broken-up driftwood cemented back together with red, yellow, and green jasper. Less than half a mile from my apartment. Anybody wanna come visit?
I'll cook bison burgers! (We even have a local bison farm and a butcher shop that carries ground bison.)