Good old Duct tape.

The groove shows up after enough time is run on it. Most of the time, you won't see it for a club race night if you pick up the carpet each time. (You may see a little by the end of the night I guess.)

Some tracks spray TC down beforehand, but it's not necessary. Also the color depends on the compound used. Tracks that are primarily Jack get blacker, but it's not any better necessarily. Just looks good for the camera!

Paul