You must see Mont-Tremblant at least once during the fall season. The landscape is spectacular with all those colors in the trees.
The colors index gives you an idea of the fall foliage. It indicates the percentage of the colors in the trees. (i.e. 100% would mean that the trees have reached the peak colors before the leaves start to fall).
For example, this picture has been taken when the colors index was at 50%: