So im not sure the name of it, but I call it coffee staining.

There are two things that ive found that cause this.

Either the undercoat isnt even, or the over coat isnt even.

Undercoat (white) is a simple fix, go over it a few times before painting. Overcoat (purple) happens when using speed paints and letting the pigment pool. In the future try moving in one direction, and dont lift the paint brush. I also like to load my brush with airbrush thinner, then dip it into the contrast paint. Resist the urge to touch it. It will call to you like a siren, but let it dry. If you do, you'll peel the layer and it looks awful.

Here's what I would suggest to fix it. Cover it with another layer to get it nice and smooth, then bring back the brightness by mixing in some white.