Whats that you say? You need to paint white over black and you hate it? It takes 40 coats? Indeed it does with GW paints, but with my searching for ways to make snow bases I have found a nice solution.
In hobby lobby, if you go to the acryllic paints section you can find a paint called Folk Arts. They have a paint called titanium white that is great. It is very highly pigmented, and is made just for coverage. It has a very bright white color and will go strait over black.
I recommend this paint for everyones pallet!