 I think I've watched one too many tree tutorials that show you how to make basically a v-shape for your branches, for a pine tree, when it's got snow on it. And that is not what trees generally look like. They are uneven. Mother Nature does not make a tree that's going to be symmetrical, that's going to have the same size gaps in between each branch. And you not only want to have the shapes of them being uneven, you want to have the colors a little uneven. So add some darks in there. Just drop some in while your paint is wet and your trees are going to look much more natural.