You want to stay away from turning the ground electrode white, which is a lean condition... or too much timing advance.
Black, obviously, would be really rich. If you get them to turn a light chocolate/tan, that's right in the ball park for a good tune as well... that would be considered more of a safe tune that'll still make good power.
Also worth mentioning: the best way to get an accurate reading on the plugs, is to shut the motor down right after you cross finish line and coast over to the return road and pull a couple plugs there to get an idea of what direction you should move towards with the tune.
Hope that helps... I imagine it will when the time comes to jet the system up past 75hp.
