The best time to come here for Fall leaf color can occur anywhere between early November through mid-December. The several weeks before and after this date range, although iffy, may be good as well.
It's not a bad idea to call either myself at Plant Information, (626) 821-3236, or the Arboretum general number, (626) 821-3222, at least a week before you come to get in idea how the Fall color change is proceeding.
Here's a nationwide animation of the fall color change courtesy of recreation.gov:
Here's some Fall color shots from years past:
