Skins grant a bonus to physical attack or health. The maximum skin level is 60.
Each titan has a Default skin that is available from the very beginning. Several titans — Moloch, Angus, and Sigurd — have a Champion's skin that can be unlocked.
To upgrade and buy skins, you need to use Titan Skin Stones. You can get them by exchanging Bronze Guild War trophies at the Guild War Shop or earn them during special events.