My mistake - The rears he ran were the Grey/Yellow. Sorry about that - Still a little foggy from the drive home!
Solid Black fronts - I'm not sure at the moment but it is possible in the future. I know at times there is trouble getting good/hard black in from the foam supplier so the thought was to make the grey/black to have the hard outer ring for the corner speed and lack of traction roll with the added steering of the grey. Also, it would reduce the amount of black we use to keep the hard black in stock and the front tires more consistant from batch to batch.