Oh man, you guys had black nightmare cars too??? I had 4 of them. After every single one, I swore Id never buy another black car, but did never the less! LOL I remember my Black Camaro. I would spend the entire Saturday detailing it, then go out with my friends or girlfriend on Saturday night. I would wake up SUnday morning and want to cry because the darn thing was DIRTY AGAIN!!!!

Ciro, next car, get a silver color. I have a Mercedes C230K now in Silver, and it never looks dirty. I liked the color so much, I painted my Ninja ZX9R the same color! LOL
As far as adding vertical "stripes" go, if I understand you correctly, you want the stripes to look a bit wet. As David stated, try using Tamiya thinner to streak the stripes downward. That should do the trick. If you want the stripes to appear wet or shiny, try mixing your pigments with a little bit of clear gloss or thinned out nail polish.

ONe last thing, if you want to simulate "water drops" of dust on the top of the vehicle, try mixing your pigments with alcohol, then dip a stiff bristle brush in the solution, then point the brish at your model, and flick the bristles with your finger. This will leave tiny water droplets all over the place.