Mutagens in the Witcher 3: Wild Hunt are quite different than previous Witcher games. There are now 4 Mutagen slots and a player can have up to 4 equipped at once. These can be changed and are not permanent. Each Mutagen comes in a color and if it is slotted next to Skills of the same color (adjacent slots) then it increases its effectiveness by 100% for each Skill slotted. Mutagen slots become unlocked as the player increases in level. Mutagens can also be used to brew powerful potions called Mutagen Decoctions.
Mutagens come in three tiers: lesser, normal, and greater. Three mutagens of one tier can be used to upgrade to the next tier. Ex: 3x Lesser Red Mutagen -> 1x Red Mutagen. There are also mutagens that drop straight from boss-type enemies, which can be found below. Note that these are similar to other mutagens when equipped, providing no extra bonus, but are unique in their ability to be crafted into decoctions. Also note that like other alchemical items, decoctions refill upon meditation with alcohol, and so must only be crafted once per playthrough.
Red Mutagens Add to Attack Power
Green Mutagens Add to Vitality