thats correct. a back cross from son to mother will give you afew red mottled birds. but most will be white, some will be black mottled. you just have to cull hard
the mating of a pure white leghorn rooster with a Red Mottled leghorn(columbian restricted mottled) hen will produce all white chicks(some may show some black spots) and mostly all white birds as adults. and no you will not produce more red mottled birds.. mottled is an autosomal(none sexlinked)...