Painting waterproof backpack?

[bobtheslinky.livejournal.com]

I checked the FAQ, please tell me if I missed the link, but here's my question:

I have the same light blue Jansport backpack as 50 other people on my campus. I'd love to jazz it up with fabric paint (don't want to sew buttons on, since that would pierce the waterproof material). Anyone know how well fabric paint (what kind?) stays on to waterproofed material? Or if some sort of sealant is required? And how durable?

Thanks!