I think what the others are trying to convey is that this would not be a normal way of saying that you want a dog that has both black and white coloring.
When you say 'a dog in black and white' it sounds as if you somehow want a dog that looks as if it's in a black and white movie (from before movies were in color). This is why it sounds so strange.
I'm finding this section frustrating. We are hearing a lot verbs, and while we've been introduce to the concept of -er, -ir and irregular verb congugation, but we have not been introduced to the roots and the definitions. Luckily, I borrowed a dictionary on french verbs, but it sure slows down the practice of using them. It's been great up to this point, but I feel I can learn this way.