Wipe and slice mushroom. Add 1 tablespoon lemon juice to 2 cups of cold water. Add mushrooms, and let stand for 5 minutes. Discard and saute' in oil or butter, or steam in a veggie steamer. Cool and freeze. The colour doesn't change.
I buy potatoes when they are on sale. Then I boil them, mash them, and seal them in serving size containers and put them in the freezer. It's so nice to be able to just take out as much as I need and microwave it.