If you've tried heating and peeling the covering, but it's leaving the adhesive/color behind, take clear packing tape, lay it over the adhesive/color, rub it in with a finger, and then peel it off. It will take the adhesive/color with it. If the adhesive/color doesn't come up the first time, repeat, but run a warm covering iron over the tape before peeling it off to get the adhesive to let go. Might make the balsa fuzz a bit, but a light sanding solves that. As always, try it on an inconspicuous spot first to get the process right.