Crossing a white female back to her split father is certainly the easiest manner to produce more whites without producing a ton of blacks of which only some will be split to White and you won't know who's split without test breeding.
My Whites had slate legs.
Mottled did show yellow, yes...but no pics of them.
The Black Java pics I have...
Looks kind of slate to me in the background?
Hatchery Australorp for comparison...
Ughh, of course, his feet were dirty and the girls aren't at a good angle. Looks kind of slate-ish...
Are you 100% sure they are Australorps? White Javas are recessive and do pop up in Black flocks. In fact, that's how they were re-discovered.
But yes, the chick is recessive white. Breeding it to another sibling that is white will guarantee 100% White chicks. If it is a pullet and you...