The light is good, and I think you need it to make it look carved. If you created the black on a separate layer it will be easy to make the highlight in GIMP or Photoshop. Duplicate the black, click the button to lock the transparency of one of them then fill it with the light color. You can change the blending mode too, whatever works. Move that layer slightly so it's offset from the black and make sure the light layer is behind the dark one. All done!