I don't think the plastic spray paint will work. That stuff creates a plastic-like coating and I'd think that would just crack when someone sat on the cushions and then chip off over time.

You can buy fabric paint at craft stores. Or buy textile medium and add it to regular acrylic paint. It keeps the paint softer so it will flex with the material and not crack. I've used it on clothes and it survives washing just fine so I expect it will hold up to outdoor weather. It may eventually fade in the sun but what doesn't? Here's a video showing how to do this: http://www.diynetwork.com/videos/painted-upholstery/6178.html If you want to get fancy you could also buy some stencils and create a design or use painters tape to mask out rows and make striped cushions.

There is also fabric paint that comes in a spray can, usually marketed at upholstery paint. I think there are a couple of different types but I've never used them so no idea how well they work. It would make applying the paint easier.

I found another thread here with good tips as well, just follow the link below.

Here is a link that might be useful: Painting fabric furniture

thanks so much alaskachris! that link had me surfing those other links all morning, it was great.