Put antifreeze in black and grey after winterizing ?

There's no need to put antifreeze in empty holding tanks. Air doesn't expand and cause things to burst in cold weather. (Even a little tiny bit of water left on the bottom won't cause problems when it freezes; it has plenty of room to expand.)

If you have excess money that you don't know how to spend, and you don't want to send it to DrewE, you can buy more antifreeze and pour it into the tanks and it won't cause any harm...but it's not at all necessary.

If you're not going to pour antifreeze down all drains, make sure you run enough pink thru to fill the traps with antifreeze.

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.

I am not equipped with by-pass, but if I were I would leave it open to allow for expansion.
One half gallon is more then I would be putting in the drains. A quart would be sufficient.
Yes, some anti-freeze in the grey and black to protect the valves as "mountainkowboy" suggested.

A good flush in the spring along with sanitizing is a good idea. Use backing sode to kill unwanted tastes.

Great thx, just dump the half gallons down the toilet and shower ??

I put about 1/2 gallon in each tank to protect the blade valves after you dump, it needs to sit in there. Yes just leave the W/H drained and bypassed. In the spring make sure you get all the antifreeze out of your lines before you bring the W/H back on line.