Cool, it's awesome that you read the book and can relate to my picture as well... it's easy to get caught up with ones own imagination and not appreciate someone else's interpretation of the source material... I do it as well.
Thanks! I actually saw the tree as some sort of weird bio-luminescent alien thing, generating its own light source. The actual sky is visible in the background and it is a sunset-pink/purple of sorts. The tree is suppose to be akin to the "Bodhi Tree" of Buddhist religion, where the Buddha achieved enlightenment.
The coloring on Buddha looks good and so does the background but I think Yama looks a bit muddy. Were you trying to go for a red ambient light with him? Well anyways if you have time I think if the highlights on Yuma were touched up a notch this would really make it pop, like always just a suggestion..