Washington Pass has a viewpoint, turn north off the highway just west of the summit. Great view from there. If you cross Rainy Pass, you're too far west.
Halfway down the west slope there are some good views of Whistler Mountain to the north. (don't confuse with Whistler, the village in BC)
Farther west there are some good viewpoints of Ross Lake from Diablo.
Much of the higher elevations is conifer, so there's not much colour. You'll find better colour in the lower elevations to the west, along the Skagit River. On the east slope, the valley bottom between Winthrop and Mazama should also be good.
The San Juans should be gorgeous that time of year. Good shooting!