a lot of young birds will fledge the nest with this colouration and it moults out in a few weeks, occasionally a young hen may keep the markings but they have never been able to breed anything from it with the markings. it is a random occurrence, I have 2 or 3 young at the moment with the same markings.
As I have been told many times by long time experienced mutaion bird breeders. Never give up. If you want to try and extablish this look. Keep these young and outcross breed them, then breed the cousins back to each other, then out cross again, then cousins again. etc. easier with free breeders like Zebs. Good luck.
LML