I think a lot of people have seen my dyed interiors already. I too tried with the seat belts (although I expected nothing but negative results). I was right in expecting nothing but bad results, cuz that was what I got. If you want to keep the stock belts, then just go buy the role of seat belt material and sow it up. Extend the old belt completely, and cut off leaving just about 8 inches of the original one, then have the new belt sown on to it. That's how I did it and it works.
A tip though. The new belts I had sown on were somewhat thicker than the stock ones, and it kinda makes the belt apparatus jam up with that safety lock thing (what prevents you from flying through the windshield). TRY your best in finding the strip of cloth as thin as possible to match the stock, or else you will find yourself nagging a bit. It works, you just have to struggle a bit more than usual, which will make you hit and swear at things on those days that you're just in a bad mood. I'll pick up the camera tomorrow and take a few shots of it in my car.