If I was hunting fall color around Glacier, I would go in early October. But there is a risk... Weather can change at any time, either wiping out the color or just closing the roads and various areas of the park. Being local I can deal with that, because I didn't invest in a trip for a specific scene that I suddenly can't reach. That said, outside of the Sun Road, nothing is likely to close for the year before mid-October. If you have several days you can likely wait out a storm and go back out exploring.
From what I have seen, aspens are generally unimpressive around most of Glacier. On the east side there are groves, but they tend to be stunted and small. I have found some decent groves around the Flathead valley on the west side. The area I plan to explore this year is the North Fork area along the road to Polebridge and Bowman Lake. I was impressed with the color on the one day I visited in the autumn, but it was late in the season. I wished I had checked it out sooner. Work permitting, I'll spend time there this year.
The larches are truly impressive in mid-October. Whole hillsides turn yellow around Kalispell and the North Fork area of Glacier. Here are a couple of examples of fall in the area...
1) Some aspens along Highway 93 south of Kalispell on 10/15. A little passed peak.
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2) More aspens right next to the highway on the same day
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3) Bowman Lake in the North Fork area of Glacier On 10/17. The larches turned the whole hillside yellow
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For the record, the time I went to Banff in the fall was last week of September/first week in October. Aspens were a little burnt but the larches looked good.
4) Larches at Lake Louise 10/2
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5) Color at Grassi Lakes 10/2
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6) Aspens after the first snow of the year (clearly passed peak) 10/3
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