Paint....

If you can remove the trim pieces, you could spray paint them with a spray that works on plastic. If you can't remove them, you could still paint them, but it's a lot more work.

Go to Home Depot and check out their faux, or wood, cornices. They should offer a choice of styles and color.
I used one on the patio verticals in my previous home and loved how it gave them a finished look. My trim was white so I used a white cornice.

If you're handy, you can make a simple cornice with straight edges and cover it in fabric.

Here is a link that might be useful: Example