Stat bonuses are colored numbers added to weapons and armors, generally crafted through alchemy. It is possible to obtain them with equipment rewarded from a quest, or by using upgrade books. There are a total of four color bonuses (green, blue, orange and gold), but you can only reach the green and blue bonuses when crafting a weapon or an armor through alchemy or when upgrading a weapon. A weapon upgrade removes the weapon's existing stat bonuses, if any, and assigns new green or blue ones.