Dye is NOT paint.
The dye does not sit on top of the plastic, like a paint. It soaks INTO the plastic.
THe key to properly dying plastic or vinyl is prep.
I like to scrub down whatever is being dyed with a 10% vinegar/water solution first. This will remove the oils & ground in dirt.
Once they're dry (next day), I'll wash 'em down again, with T.S.P. Paint stores will have this. Comes as a powder. Mix as directed on box. Use a good stiff bristle brush for both of these procedures.
Rinse, rinse, rinse, with cold water. Let sit to dry in the sun, or somewhere warm overnight.
Do not touch anything that is gonna be dyed, after these steps, with bare hands. Use 'throw-away' rubber gloves, or touch only where it won't be seen. The oils in your hands will effect the adhesion of the dye.
Hang small parts on coat hangers or wire.
Several light coats are better than a couple of heavy ones. Drips will NOT be able to be sanded out.
Let dry for at least a day before handling.
Good luck.