There's a post here about the art design in Up and there is a good section about the forest, I really like the stylised shapes of the trees so maybe we could use this as inspiration for our own? There are also some videos of lighting tests which are interesting to see.
For the forest I like the idea of it being hot colours instead of blues and greens because it is more unusual and perhaps it could be so hot it almost seems to be on fire, or it starts off golden and gets redder and redder as the character goes through it?
For shapes maybe a bamboo forest would work? I imagine it would be easier to pull off than having regular trees which would probably be quite hard to get looking decent:
Here are a few photographs of ice and snow which have a lot of nice colours:
And I thought this black and white snow cave picture has a nice feature if we are using a cave as the portal to the ice world because a hole in the ice might give us the chance to do some interesting lighting.
Purple and blue look pretty good together:
As do green and blue- this is from Lou Romano's blog, http://louromano.blogspot.com