Some biomes should have clouds blotting out the sun. some biomes should have no clouds at all.

Add color to the sky in specific biomes where appropriate. Add more/less clouds depending on biome.

This is on the features list for a future release.

Good luck. Seriously, good luck.

Twilight forest has a custom sky renderer, so that even at light level 13 it is dark and you see stars. Optifine apparently does not use custom sky renderers, and the world looks very bright.

On the mystcraft forums, when I had suggested the idea of a biosphere like "Age sample sphere" world, someone responded that trying to have custom sky rendering based on location would be a nightmare. I responded that I was only thinking of the ground/block layout, not the sky, but given that others have already looked at that, in a mod that has support for that, and concluded that it is a pain ... good luck.

heh...thanks!

Actually, Forge just added a hook in the sky rendering code that make all of this very simple.