I think that fashion "do's" and don'ts" exist, but usually they are a matter of severity. For example, if you were wearing super low-rise jeans with your thong underwear intentionally hanging out the back, then you have just committed a rather obvious fashion don't. So many things pass as fashion do's these days that I really don't consider things like "is it okay to wear a navy bag with black clothing?" a valid concern. I would consider it a petty complaint from a very shallow person if you were out and someone actually cut you down for wearing a navy bag with black clothes....