See the ink on the tires in the link below? Was done with this ink pen. It's an opaque white ink, that you kind of 'roll' and dab into the lettering.
Works better than anything I've seen, and is pretty fast to do a good job. It even comes in colors! I have neon green, yellow, red, and neon blue for use whenver I want.
In fact, the link below, i've followed up and made the lettering neon green to match the paint!