I’ve been watching the fall color the past 2 weeks over in the Eastern Sierra—it is coming on strong at all elevations right down to 6000', as I saw today. It seems fairly uniform from 6000 to 9000 feet, all going at once, probably due to the the hard winter-grade freeze 12 days ago which dropped temperatures down into the teens for two days at 10,000'—I slept in my Sprinter photography adventure van that 21st and 22nd, and it was 22°F inside the van.
Yesterday and today were much colder (a drop of 10°F in overnight temps with just above freezing down to 6100') and I encountered snow squalls whith accumulating snow at 7800' elevation driving back through Yosemite. It was already sub-freezing at 10,000' at 4 PM. Those aspen are not going to wait for latecomers, it seems.
So if you are planning on photographing fall color, I’d say try it this weekend for some already nice stuff in places (and green/yellow is very interesting too). Two to three days and there will be some very nice color, it looks to me that by about a week from now it should be going strong, unless a powerful windstorm rips off the aspen leaves, as is not uncommon.
Image from 2016— many leaves ripped off by windy storm.