With our British heritage but the influence of our neighbours to the south you will see both autumn/fall, colour and color being used fairly interchangably.
As for the best time to go it depends if you are looking for a specific tree/bush species or the overall effect. Being nature it is next to impossible to pick out exact dates well in advance. It depends on temperature, rainfalls, water levels, wind and other factors. Generally between mid-September to mid-October you are likely to see some colour, often mainly yellow. If you wish to play the odds then the last week of September to first week of October is likely your best bet but we truly don’t know and certainly don’t make any guarantees.
Some 3rd party advice on fall colors in the Canmore/Banff area
Difference between a Larch and a Tamarack – same genus, different species.