I have no clue!! But I can deduce as much as I can for you!! ;)
So, venomous snakes always have two distinct properties: they are brightly colored and have the heart on their head (where they keep the venom). My guess would be that the king snake evolved to have bright colors so that other predators would think that it is poisonous and stay away.
Th resulting phenotype would be homozygous yellow flowers. This is because one homozygous yellow crossed with another homozygous yellow, so of course their offspring would be the same as well. Hope this helped some :)