ive made marbles.I just pretty much bred the black out of them.I started of with a black male n pink female.bred them together got babies,some pink some black.I took a pink female bred it back to the dad,some black some pink,took his pink granddaughter bred it back to original black male got babies.Took a female pink from that batch bred it back to the origianal black male.and so on.Now during that time i was also breeding the pink males back to the pink female back to the mom(pink) resulting in all pink.ive got marbles both ways.evenually you will get marbles this way but it will take some generations.This method has worked for me many times.Im thinking of trying to make some marbled leucistics at the moment but we will see.hope this helped.