Don_K
I am thinking of air brushing muiltiple light coats of Flat White over the stencils. Is this worth trying? If not, then I guess I could try Testors enamell Flat White for the markings.
Airbrushing is going to be the best way to go in most cases of applying one color over the top of another, especially when using a stencil. When brush painting, the paint is going on really thick and wet and will have a tendency to creep underneath the stencils. That wetness and thickness also leads to what you saw because not only are the solvents in the wet paint being given more active time, but you're also applying a mechanical stress to the paint underneath which causes the mixing you're seeing. I haven't found any hard, fast rules for what can go on top of what when brush painting. It always seems to be hit and miss, except in the case of applying enamels over the top of a real lacquer like MRP, Mr. Color, or Tamiya's Lacquer line. With those 3 paint types, they are all pretty bullet-proof when it comes to an enamel being able to dissolve or damage them.
With airbrushing, you don't have to be concerned as much with paint creep either. When airbrushing, you'll just want to avoid angling the airbrush to point into the edges of the stencil so you don't get paint ridges. I highly recommend getting comfortable with masking and airbrushing things. It just looks so much better.