The season runs about Sep 25 – Nov 5, and it varies year to year; this page's reports say what the road shows this season.
Which exit for fall colour on US-2 and I-90?
From the road: Snoqualmie Falls (exit 25); Rattlesnake Ledge (exit 32); Franklin Falls and Denny Creek (exit 47); Lake Easton (exit 70); Gold Bar; Index; Skykomish.