There is another thread for this topic already. I don't know how to link you, but if you go back to the "Civ IV-Bug Reports" forum and look around, I'm sure you will find it. It is called "Terrain is Dark" and it was half way down the list last time I checked. It is about 5 pages long now, and there are some suggestions on it for how to fix it, though none of them work for me (yes I have the same problem). Maybe you'll have better luck.
To answer your question, it seems like there is some programming bug. We are all anxiously awaiting a patch.