I've used the Black, Gray and Blue, don't really see any difference in them on my 99, presently have the Blue in now. Can't get the Black anymore, at least not new ones. There is a difference in length between them, Blue is closer to the "gear" which advances the timing a little, so I've been told. Couldn't see any increase or decrease in MPG between them.
You know what? You can actually use a CPS for a tach pickup on the IDI, just thread the end with a 3/4" bolt die, screw it in with teflon tape on the threads til it bottoms out, then backoff a 1/4 turn.