This year I haven't been to scotland in autumn, which is a shame.
I stumbled across tutorial that show how to make an autumn pic out of summer one, and given the right type of picture, it looked easy, so I gave it a go using a picture taken in late spring pic on loch lubnaig. Basically, it's just selectively adjusting the hue for different colors (mostly touching yellows and greens), plus a few more tweaks
(Interesting, it looks to me that photobucket hosting site eats into the color somewhat)
here is the original version (click on a magnifying glass on the top left of the preview to see full size)
Not every type of pic seems suitable, though. This pic from Glenfinnan immediately starts falling into a mosaic on obviously spring (water, distant mountains and sky) and fake autumn