Why is it in black and white?
Director David Slade explained it in an interview.
I found Charlie to be incredibly open — very, very trusting from the get-go. I quickly signed up, and within a month, I was there with them. I kind of waited until I was in England before I was like, “Look, let’s shoot some black and white.” They responded very favorably to that. They did some tests, and eventually we were like, “OK, we’re gonna commit to black and white.” Again, the location photographs I was taking as I was going around, the one thing I kept coming back to was the oppressive nature of the story. So we shot it on a native black-and-white camera. Once we made that decision, everything switched to black and white. My memory of the entire thing is in black and white!
Also from the interview with Charlie Brooker
Why did you shoot black and white? Was it just to be evocative? Or did it also save on CG costs to render the dog?
That was the director, David Slade. He wanted it to be black and white. Like you say it does put you in mind of old horror movies and it fit with the sparse, pared-back nature of the story. I don’t think it saved money on CG. It felt like something I hadn’t seen before — doing lots of CG in black and white.
I have a personal interpretation of why the episode was shot in black and white. The story line is set in a post-apocalyptic world where intelligent machine dogs attack and kill humans. Now the black and white could be because of the fact that dogs themselves are colour-blind. Thus, it can be shown that the rogue dogs are ruling the world. Or maybe I'm just reading into it a lot.