Can I slice potatoes with the skin on, wrap in foil and keep in the refrigerator without the potatoes turning gray or black. I will be grilling them 4-5 hours after prepping them

I think this will work fine. A little clarification needed, are the potatoes going to be wrapped up with onions or by themselves in aluminum foil and the packet then placed on a BBQ grill? When slicing the potatoes place into water with a little acidity to keep them from turning gray. A small amount of white wine vinegar of lemon will do the trick. Then drain and place the potatoes in foil along with your spices and some olive oil or vegetable oil. Wrap up tight and refrigerate, then grill. Or are the sliced potatoes going on the grill directly? Do the same procedure, but wrap in plastic wrap and refrigerate. Then remove to place on the grill.  Another option is to microwave the sliced potatoes till tender, cool and then refrigerate.