Fix static analyzer warning for "NSMutableDictionary key cannot be nil"

This is my first contribution to open source so I hope I did it right.

Looked like a simple fix, and I think it is functionally equivalent. I'm not sure if the Color Pack and Color folders were supposed to be the same, but I wasn't sure which implementation of the category to use.
But that is why I was a good boy and did small commits.

Maybe I could add a test target and write some unit tests for that?