I did mine with Undercoating paint. Got the idea off of a Corvette forum. It blended very well with the tire. Very pleased with the end result. Spray the paint in a cup and apply it with a sponge brush.
I've also colored it in with permanent magic marker then went over the whole tire side wall with "Harley Black" tire dressing to make it even colored, worked great.
Thanks guys, I have to order a new rear tire, I was going get a black one because the white stripes are a pita to keep clean. Only problem is I have a new nww front tire installed last fall, so I was looking for ideas to get rid of the white wall.