I hate doing white but in my hate have found two methods that work very well.
Reaper bone in layers gives a very good almost egg shell white.
The other way is yellow as a base and then layers of white.
All depends on how white you want, worn you go for less for storm trooper white you want bright bright white. That means lots of yellow under and pile on the white. The other trick is make sure the detail lines are super dark, so they contrast well with the white.