Welcome to the Gauge Rich.
Paint color is a real subjective thing. Like others have said, take your cue from Mother Nature. See what the real world looks like then mix up some colors to match. If you get it wrong, or don't like the results, add another coat to whatever you're painting.
As far as trees and plants, you can buy them ready-made, or make your own. You should go to your local hobby shop, or possibly your library and look through the book section. There's a bunch of good books out there on how to make scenery, including fall.