You mean a specific set of colours? You get to choose two progenitors when you first start out, those are the only really exact ones you can make. For everything else, you'll either have to get lucky and find someone who is selling what you're looking for, or find a breeding pair with as close a range to your desired colours as possible and breed them to get what you're looking for.
Oh! Merle is a gene, you can find those in the Mole Market - however, I think this particular one is a moonstone gene, so you might have to save up a bit.
