If you use SEM products correctly using their cleaners and prep products you can do a perfect job the first time. Hard plastic is easier because it's easier to clean it better. They have three different blacks in their Color Coat line of aerosols depending on if you want a shiny, satin, or flat finish on the plastic.
The secret to dyeing hard plastic is to use adhesion promoter. This allows the dye to be pulled right into the plastic. SEM has three different adhesion promoters for different plastics, so be sure you use the right one for the plastic in your car. If you look on the back of your plastic pieces, sometimes it will tell you what plastic the parts are made of, but they have a test to let you know which adhesion promoter to use. Here are the instructions right from SEM's web site:DYEING PLASTIC
The picture shows two different adhesion promoters, the other one is SEM XXX adhesion promoter.