Some colour maps are not "premultiplied" by the alpha. Even worse, some are on white backgrounds.
Ideally your colour map should be on a black background, and the soft edges on the leaves should fade nicely to black. If so, "unpremultiply colour" should be ON in TG. (In some cases your image might not really be premultiplied correctly, and it might look better if you turn off premultiply anyway, but expect the edges to render dark in TG in that case.)
If your colour map is on a black background but appears to have non-antialiased edges, or the whole background is filled with colour, then in TG "unpremultiply colour" should be OFF.
If your colour map is on a white background, turning unpremultipliy colour OFF will help, but to avoid light-fringed edges you'll get better results if you replace the background with black in a paint program.