Would this mean that he is not actually D'Uccle? Here is the pair.
He might be a d'Uccle (breed). But he is not from a flock that breeds true for any one specific color.
Since he is obviously carrying one gene I didn't expect, I can't predict what other genes he might be carrying. For all I know, he might already be half porcelain, or carrying a mix of genes from several different color varieties.