Bag-cooked omelettes turning gray when stored in the fridge
Cooking, not storage, to blame
Storing cooked eggs under refrigeration for a week is not turning your eggs gray. Instead the discoloration is from the iron in the yolk reacting with the sulfur in the albumen at temperatures of 170F or higher creating ferrous sulfide.
Overcooking the omelettes initially, or perhaps hot spots from the reheating method, create the gray-green discoloration. While unappetizing, the discoloration is not harmful.