Pardon our redundancy.
As everyone says, black first, with a few refill and dump cycles, and then grey.
I have potable fresh water hoses and non-potable fresh water hoses (which happen to be old white hoses that I have striped with red Sharpee and clearly marked "Non-Potable".) After dumping, I rinse the stinky slinky with the non-potable hose, drain it well, and put it all away.
Potable water hoses and accessories (pressure reducers, whole house filters...) are stored in a separate container, with the lid and bottom perforated to encourage drying, in a compartment apart from the dump valves.
