Mottle and Mille Fleur are based on the mottle gene (Mo/mo), with additional modifiers to enhance or restrict color. Since it is recessive, meaning that birds with mottling are mo/mo, it could easily be removed visually, but not so easily genetically. you could have birds that carried it but did not appear mottled (Mo/mo). It could be removed similar to the way white legs can be bred out quickly by crossing to a yellow-legged bird. So if you had and excellent hen, she could be crossed with a normal-color rooster and would be expected to have chicks that did not show a mottle pattern.