The purple is there because it looks more vibrant than black. This technique is done in actual paintings as well. I think it all looks great and the style is fantastic. If I were you I would take critique on actual technical issues and take no critique on style changes.
I really don't think the vibrancy of the purple is its' reason to exist, black can be just as vibrant, and if the purple were made any more vibrant, would it become more 'successful'?
It's on the trunk too, the idea is that light is being bounced off the ground and is lighting the underside of the tree a bit.
And I'm guessing the only way to explain it is that the tree is casting a slightly purple shadow, and light is being bounced off that shadow
The invaluable reference that's been posted a million times before:
http://www.itchstudios.com/psg/art_tut.htmand the cliffs look good so far, they're just lacking depth
at the moment they look like a texture wrapped on some flat walls