Hey, red would give you pink mashed potatoes - very festive, and just in time for the holidays. How about making one batch with red horseradish and one with green wasabi? You could pipe them in patterns together. Nothing says Xmas like horseradish mashed potatoes!
Horseradish loses its strength in a few months, whether fresh made or jarred. I'm a big one for using every last oz of everything, but after a short while, even if not opened, it loses all its potency.