I'm helping my daughter paint the exterior of her home and one of the things that needed painting was the shutter's made of plastic. They were in perfect condition; they had just faded a little, so they needed a fresh coat of paint. Up until recently, there was no paint that would adhere to plastic very well. But now there is! RustOleum has a spray paint specially made just for plastic. It doesn't even require a primer and can be used for interior and exterior applications. The semi-gloss finish is washable.
My daughter's shutters were originally maroon colored, but she wanted to paint them a dark green. She purchased 3 cans of the dark hunter green and it covered 8 shutters with two coats.
After power washing the shutters and letting them dry throughly, I just lightly sprayed each shutter, so as not to leave any drips and then about an hour later re-coated to give a nice even coverage.
The shutters now look brand new!