If I had bought a new van, I would have made new wooden window frames, but since the existing frames are still in good order, I have decided to paint them. Last week I bought a spray paint specifically for plastic.
Unsure whether it would hold on the frames, I tried it out and the results were great.
The bottom of one frame has to wait until I have a fresh supply of paint. I calculated that I needed 2 or 3 more bottles to finish the job.