What S The Difference Between A Frostbite And A Purple Bruise?

What s the difference between frostbite and a purple bruise? The reason for that is because I twisted my ankle and it got swollen and the swelling went down but it got purple around my ankle but not a big bruise just like a line on the side of my foot and i don t know wether it s from the ice or from what occurred with my ankle? And by the way I put a sock on before I applying the ice