The trick with preshading is to not cover 100% with the primary color, but still not too little. The preshade should only barely be noticeable. To achieve this, you must use very thin paint. There are other techniques to achieve this effect, but preshading is the most common.
Flat acrylic coats can be troublesome, but not impossible. Get a brand that you like, and try to put it on wet, and it will not turn whitish. I believe the most common problem is probably that it start to dry before it hits the surface, and makes minute air bubbles get trapped in the drying process.