@ Daklunk : Sorry to ask this, but what is the "axle halves"? I ask cuz my english is ok but not that excellent and because my knowledge about cars is quite short ^^. Anyway I remember having read something about a film that you put in the water and the longer it stays in it the darker it becomes : it's called RIT brand fabric dye.
An other method I found is, a says energon, to use some tamyia smoke and an airbrush. Thing is I don't have an airbrush and they are pretty expensive too (I saw some revell ones for like 20€ but :-s)! But I think I'll leave the windows tinting out ;-)
Anyway, thanks for the support!