Can't rename, remove, or add mail category

Version 7.0.26687.0

I’m trying to rename a mail category (one of the default “color” ones)  to “Personal”, but it says it can’t because that is a duplicate name. But, there are no other email categories named that. 

Also, the remove button is greyed out, so I can’t delete them either. Am I just missing something, or is the mail category functionality really that crippled?