Put antifreeze in black and grey after winterizing ?

I never put any stuff in either tank. I dump them PRIOR to flushing then by pass the HWT, once the pink stuff is flowing into the drain. I just close the pump valve and wait for spring. I figure the water being pumped out for the pink then the aMOPUNT OF PINK GOING INTO THE TANKS SHOULD HOLD FOR THE WINTER. Don't forget the outside shower or the outside water valves (I have 2).
We have been doing it this way for many years with never a problem.