Maus 2- first half

One of the things that I found most interesting in the book was the masks. In our discussion we talked about what the different animal masks represent and how the cat/mouse thing was chosen to symbolize the relationship between those two animals. However, I do not think we fully answered why you could tell they were wearing masks in the second half of reading and not the first. I was also wondering if there was a reason that Art said he didn't want to be a bunny in the beginning. A bunny symbolizes a similar relationship to a cat as a mouse does but Art makes it clear that he doesn't want to bunny. There are also some mice that are black instead of white. Is this for organizational purposes or for some other reason?