I've been looking for an easy way to paint 'naked' trees. Haven't yet found an alternative to a script liner for close up trees and it takes more time to paint each individual stick and twig than I consider fun, but for background trees where detail is less important I have found a cheat. Result in my gallery.
My new "paintbrush"
(While the site is in monochrome, right click the image and select Open Image In New Tab for the colour image)