Need some help with testors enamel

Ok so I've never used Testors paint before.  I use Tamiya and Citadel.  I'm painting a couple of cars with my girlfriend though and the colors were just better.  I primed both in black and then when I realized how dark it was making the colors I put down a coat of siver on both cars.  I painted mine in plum crazy purple and she did hers in red metal flake.  I have several problems.  The purple seems very uneven at times and kind of think and not leaving a nice coat.  The guy at the store said Testors doens't need to be thineed to hand paint it.  Any ideas?  Now, on my GF's car I did thin her red, thinking it would help.  It wasn't watery or anything but as she was painting I noticed an issue... It was taking up the silver paint and leaving the black showing.  I told her to switch to out of the bottle and figured I had thinned it too much, but the unthinned took up some of the silver too.  What gives?  Anybody have some advice for me?