part of the exclusivity of javas is the pure blood, not mixed with other breeds. They did studies on flocks to see what were mixed heritage and which were pure java and went with the pure java flocks to propagate them again and get them more available to the public. I think mixing them with another breed as well as changing their combs would make them not javas any more. You could fiddle around and just breed a bird that suits you though, just don't call them javas. That is what I am doing with my barred olive eggers. They look sorta like Ameraucanas, but are as far from that breed in egg color and shank color as possible. I don't call them barred ameraucanas though, because they ain't.