It is a result of not being pure for dominant white.
For example, cross white leghorn to black australorp(this is exactly what Austra Whites are), you get cream chicks with random little black spots on the chick down. They either grow up all white, white with random partial black feathers or something like off white color with black flecking.
In the cornish x chicks it is either a result of cross involving non-whites in the breeder stock or is simply not pure white "floating down" the generations.
as it happens, got two CX chicks from feed store last fall for breeding. Both had little black spots. Just hatched a bunch of chicks from the cockerel over other hens(not cx) and they prove he was not pure dominant white, as half of the chicks came out white with the black spots and half black chicks with white head spot-barred, as in barred rocks.. barring is often added to white chickens to help with clearing the skin and legs of pigment(makes their legs yellow instead of black).
sorry can't answer the carcass skin part yet. I'd guess for most of them, the skin should be a clear yellow.
Edited by Kev - 4/27/16 at 11:22am