Atomic Rabbit's name was changed to Atomic Bunny with Issue #12, which kept the rabbit's fur black. But, it is clear that the changeover to having his fur gray on the front covers was a premeditated editorial policy change, starting with #13, and lasting through #16, with it reverting back to black at # 17, but changing back to gray at 18, and then back to black again at #19 (the series' final issue).
The use of different colours on the lead character's body or clothing on the front cover from black used on the inside pages was reasonably common in the comic book industry. Donald Duck's sailor jersey (shirt) was black on the black and white inked comic strip pages, which preceded that character's appearance in comic books. So, that tradition was kept up later, when his full colour Sunday comic strip was started, and kept the same when his strips and stories were first started in comic books. However, the front covers of comic books were the eye-catchers for potential readers, and so, the glossy, shiny bright colours of the new books would catch the eye best, to make them stand out on the magazine shelves. So, comic book publishers often changed the large blackened portions of their characters' clothing costume on their books' inside pages to a lighter, brighter colour, on the books' front covers. That was especially true for funny animal genre characters. I admit that it is more unusual in the case of Atomic Bunny's fur (rather than a large piece of clothing being changed). But, I'm sure he wasn't the only black-coloured lead character whose colour was lightened and brightened on his front covers.
Link to the book:
Atomic Bunny 15
