I live in Denver, CO and my Autumn Purple Ash tree has leaves that are turning yellow in the last week of July. Tree was planted 8 yrs ago and haven't had any previous issues.
Autumn Purple Ash
Certified GKH Gardening Expert
This article will help: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm
This article may also help with identifying disease problems that could cause yellow leaves on ash: http://homeguides.sfgate.com/yellow-spots-leaves-autumn-purple-ash-45245.html