If you meant the foreskin, then it supposed to be same color with the rest of the skin that covered the shaft.
But if you meant the head, well...usually it's color are pink. Some are light pink and some are reddish pink. It's all depend on the person.
However when that person have sex intercourse, which caused the foreskin to peel backward, or have his penis circumcised, which caused the head to expose to the outer air for a longer time, the skin on the head then gradually turn darker color, due to the oxidization.
But that just me theory, without any scientific backup to proof that I'm right. :P