If you want more mottled birds, you need to breed with a mottled. Mille fleur would work also, although the pattern will probably not be as refined as a bird carrying two copies of Pg Ml. If you can;t find a mottled or mille fleur, breed to a black, then breed those offspring back to the mottled mama or to each other. Since you are working toward a specific goal, you should not breed to recessive white--you never know what genes that bird will be introducing that can mess up the pattern or colours.