I have spent 40 consecutive years in Grand Teton/Yellowstone National Parks and worked there as a Park Ranger, in the 70s. I might know a little about those two parks and would be happy to share some of this. There are a lot of factors that will influence what will impact the fall colors there. Mostly, wind and snow. Things begin shutting down (campgrounds/services) in the Tetons/Yellowstone after the third weekend of September. Gros Ventre (pronounced; Grow Vont) Campground will be the only campground open in the park, into October. Snows can happen anytime, and begin to increase into October. Expect temps, 60s-70 daytime and nighttime, 20s-30s. The best part, inclement weather moves through quickly, so just be patient.
I did a trip report many years ago and much of it is still relevant. The photos no longer work, as the RV.net script corrupted the tags on them.
Tetons/Yellowstone.
If you would like, send me a PM (as anyone can) and I will send a “cheat sheet” of hikes and places to visit and eat.
As for the Photos, start looking for the color changes around mid September to the end of month. Colorado usually is the last week of September to the first week of October.
Have fun!