The start of autumn is usually quite insignificant. A chill in the air at the beginning and end of the day and condensation on the car windows in the morning don't really change too much for the plein-air painter. Apart from being able to ditch the suncream, life continues much the same as usual with the addition of a few extra layers as the weeks pass.
Autumn, of course, is all about falling leaves. Our American cousins definitely hit the nail on the head with the Fall but the strongest colour in autumn takes quite a while to develop and indeed it's not until around the end of October in the UK