On Saturday, I made purple mashed potatoes, see pc. They have a distinct purple color when fresh. However, when I went to put leftovers in my lunch this morning, they were very blue. This is the second time I've refrigerated leftovers and had a color change. The first time, I didn't really make a note of it, but now it's got me wondering... Why do purple mashed potatoes turn blue when you refrigerate them?