As alluded to above, the smoke from forest fires may be your biggest worry. They have been impacting that area for the last few years more or less regularly during late summer and early fall. As for fall color, either of the weeks you mentioned are reasonable bets--it comes a bit earlier in Yellowstone, which is higher. Do take note that some Yellowstone Park campgrounds and lodging close down at the end of September, and some services get sparse/limited a little before that as the summer hired help starts exiting the area.