Google images shows cabs with both red and yellow. (Mostly. There was one pic with white, too.)
The KC flyers don't show a cab, so that won't work as a reference. I don't know where else we could look, short of finding one in a vintage video clip or something.
Personally, my brain wants to say the ones I played as a kid were yellow. I think red looks better, though. It seems unlikely that ops would have changed them over the years (especially all to yellow), so it seems reasonable that they could have come from the factory with multiple colors.
So I would tend to agree with DLP, the answer is probably any of the above.