I use the original Terragen, and you don't need professional to make terrains green.
Just go to Landscape - Surface Map. Double click on Surface Map, and there should come a new window with information about the terrain (roughness, etc.).
Then click on the button that says Colour, and a RGB panel should appear, which will allow you to adjust the terrain's colour.
Also, you can add various colours to the terrain by using the 'add child/sibling' feature.
Thanks, Man of 1,325 Pets. (who could ever have that many pets?) Anyways, I did have a quite awesome b-day. Thanks again!