I imagine if you covered the rest of the tire with masking tape except for the sidewall portion, you could pull it off with a good can of spray paint. They even ake white tire paint specifically for this purpose, but it's brush-on stuff. Doesn't sound bad for $9 a can:
http://www.rangerpaint.com/
I had some white overspray on my tires from painting the frame, and used black spray paint on my old tires to cover it up. You can't tell the difference. But that Ranger Paint stuff sounds pretty cool. Might have to look into that stuff eventually.