Seems that black sells best, but I think right up there with them are the stocky calves that are clearly heavily Charolais influenced.
On a related note, at the last two all breed bull sales I attended the black bulls (of all breeds) were clearly more in favor than comparable quality red or brownish bulls. And the Charolais were definitely selling cheaper than they were a few years ago when they were all the rage. It is really interesting, and I'm sure disappointing to the "red" sellers, to see black Limo, Simmental, Angus and Brangus sell so much better than comparable quality red bulls of those breeds.
I attend a bull sale in El Campo, Texas each year and there is a fellow from Adams, Nebraska that usually brings about 40 nice bulls to sell. Some years ago he brought mostly crossbred bulls that were 50/50 red Simmental/Hereford. Now he has switched and virtually everything he brought to the sale last weekend was 50/50 Black Simmie/Black Angus. As I recall, the only exception was one pure black Simmie and one black Angus/Braunvieh cross. I guess you have to do what the market tells you to do!