The color genetics would apply to using a cochin roo too! I had a cochin roo who was truly the BEST roo I've ever had. Super silly fellow. Very terrible at getting the deed done though. Too much butt fluff, not enough grace.
I'd say a blue or black brahma, because if you breed a half blue (or black) and half lavender bird BACK to either a half and half or a lavender, you'll get half the chicks with lavender and half black or blue. First generation bred to a black or blue rooster will be all be blue or black though...