[Tamar]

Hello Jeremy

A lot of good advise has been posted, here my 2 ct's

In general vacformed interiors are "sucked" from transparant or black PET, Lexan is hardly used anymore (too expensive)
If transparent a plastic primer is your best base, can be a very thin coat.
I usually use thinner based paints for my base colors and waterbed acrylics to highlight the details. This way small errors can be corrected.
If using very small brushes is not your forté you can also add detail with fine tipped water-resistant markers like this from steadler.

   
I.m.o. painting a vacformed interior is not a matter of "coloring within the lines" its more about creating a suggestion
As you can see in the image above the first step was to highlight the bright color in basic black & white.
Below I added some minimal touches of color with paint and markers and then added washes of very diluted acrylics to create shadows and accentuate shapes.

   

The best tip I can give for painting vacformed interiors...keep track of what will remain visible once mounted in your car 

   

with kind regards
Tamar