He has a porcelain parent. Porcelain is mottled lavender buff columbian, so it’s the only one who could’ve given him his buff pattern. Lavender mottled is just black under the lavender.
For your project, do you want to add mottling or just keep the buff cuckoo coloration? This cockerel does not...
What other varieties do you have in that pen? He is barred (cuckoo) buff columbian. Mille fleur, for example, is mottled buff columbian. Do you have any porcelains?