That’s pretty much the method I use (as taught by Aggro in a neat tutorial seveal years ago on the forum), only instead of using green paint, I use a mixture of grimy black and boxcar red for the trunks, then spray it with cheap hairspray, dip it in the ground foam mixture, tap off the excess and spray it again. That hairspray really WORKS! I mean, it allowed women to wear those Bee-Hive hairdo’s in the 'sixties that you could bounce a boulder off of, so it sure holds the trees together, LOL!
Sometimes you can find similar materials in the cleaning sponge department. Could be Scotch or any other brand of natural fibers. They can come in a variety of colors.
I’ve used toothpicks or short BBQ skewers to make smaller size bushes & trees. Next to the 10", they may look like seedlings or shrubs. THe fun part comes from cutting them with a good pair of scissors then adding a bit of WS grass to change the color.
Edit: Look for filters or cleaning supplies made from celulose. Cut it in cubes to fit your toothpick or stick. size can vary according to scale or what size trees you want. The Furnace filters can be washed & reused.