I just did a stump cake this past weekend. Ihad four bowls of buttercream icing that I colored dark brown, a little lighter dark brown, a dark gray and a gray. I let the icing set overnight to darken. I then put large spoonfuls of the colored icing in to a bowl and slightly mixed them with spoon. Then I put the muddled (yes it didn't look too pretty) into a 16" pastry bag with a #21 tip. I then piped up and down the side of the cake, never doing a straight line. Sometimes I would pipe in a circle and then around it. After getting all the way around the stump, I took a paper towl and smashed down the piping slightly in places. Real bark on some evergreens have a rough look to the bark yet it is kind' of smooth too.