Why WOULD you say that? What part of the world do you live in to frequently hear it said that way? It's just not how it's said.
I've no idea why a native Chinese speaker might suggest such an abomination. Perhaps you could enlighten us?
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.
And I would agree with you. As I am sure you would agree that being sharp with a fellow learner is not only unnecessary but ultimately unproductive. These pages need to be open, honest and civil.
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.