I have a perlino mare and sold a light golden buckskin stallion, their foal is a buttermilk buckskin. The same stallion crossed with a bright golden mare had a more golden foal. I think that one parent would likely need to be the lighter color (buttermilk) to have a better chance at getting a buttermilk. Just as breeding a smutty buckskin gives one a higher chance of having smutty buckskin foals. JMO.