At first blush, I thought "black" ... but let's talk about clear. Is there any performance disadvantage to clear? It would be really nice to be able to check the rotor phasing with a timing light and a clear cap. If we move to HEI style female-end wires on the distributor cap side, and the caps are black, we would need two caps ... one with the hole in it to check rotor phasing and one for normal duty. A clear cap would solve the two cap dilemma. And I suppose you could see the carbon tracks as they build up on a clear cap. I'm on the fence ... what are the technical arguments?
BTW - the Robb/MC adapter is gone ... I tried to order one and Robb canceled the order and told me to search on "firecore" .. nice guy that Robb!