I've not done panels or seats, but I did dye some vinyl sunvisors and map pockets several years ago with Plasti Color Ultra Vinyl spray. Easy to use - just like spray paint. I had a '78 F100 I had changed the seats and door panels from tan to blue and was trying to match the visors with the door panels. The color was lighter than what I hoped, and I never installed them. The pieces I sprayed are still in my garage and look as bright as when I sprayed them. They've not been exposed to the sun though.
Do not plan to install any parts you spray with the Ultra Vinyl anytime soon after you spray them! They dry quickly, but the fumes are very strong and nasty, and it takes a while to air out. You do not want to spray that stuff in your basement! I used the Ultra Vinyl probably 10 years ago, so there's probably other/better products out there now. Krylon and others have paint designed for plastics and vinyl now that weren't out back then.