According to Wikipedia (under Solid White (chicken plumage)), La Bresse chickens are dominant white. So in the present case the chickens should be white or white with a little soot. Interestingly it said that chickens with recessive white tend to be 4-10% smaller than their non white counterparts (we've probably all seen this). It goes on to say that the main meat breeds in the U.S., the white cornish and white rock, are both recessive whites. They didn't let their genetic shortcomings hold them back.