Leghorns are commonly used in layer breeds and they are dominant white. This leads to breeds with paint coloring (one dominant white gene, one black gene). Also, Smokey Pearl are known for ghost barring, another sign of dominant white.
Looks like your white must be dominant white, creating paint. Which is weird because Cochins only are recessive white and a quick Google search brings up no paint Cochins advertised and I know people would be all over that. If your white really is dominant white, your mottled rooster with these...