I can't say anything authoritative about the Boulder area, just that aspen on Boulder mountain do not turn all together (at least I've never seen them turn yellow together), so it looks spotty in the sweeping shots. Like all aspen you can get nice images of individual groves (which stand out very nicely in fields on the eastern flank of the mountain).
In mid to late October the cottonwoods in the canyons turn a bright yellow, and I think they look just grand against the red rock. The lower end of Long Canyon (along the Burr Trail) has a beautiful grove, as does the Escalante Canyon where Highway 12 crosses (nice photographic vantages can be found along the highway to the south).
The Wasatch are the north-south running mountains just east of the main population corridor of Utah, providing a lot of recreational habitat for the city-dwellers there (of whom I am one). They run from the Idaho border down to Nephi. I think the fall drive on the Mount Nebo Scenic Loop (from Payson to Nephi, the long way around Nebo) is a pure delight, very photogenic, with many good photographic vantage points just off the road, and even more if you hike the trails. Very popular, though, and the place gets crowded on fall weekends. Neat thing is the show up there lasts for about a month, changing almost daily. Nephi is about 150 miles north of Boulder, but the roads north to Nephi take you past the fall show on the west flank of Boobe Hole Mountain (below Fish Lake, great in afternoon light) along Highway 24, and then along Highway 89 through some very scenic small Mormon farming towns (irresistable to anyone with a sense of nostalgia).
Further to the north in the Wasatch is the Alpine Scenic Loop, but that costs money to drive ('fee demonstration area' they call it), and the fall scenics aren't so grand as behind Mt. Nebo.
