The Witcher 3 mutagens

Activate mutagens by placing them in the diamond-shaped slots on the Character panel. Next to each slot is a set of square Ability slots.

Activated mutagens grant special bonuses to your character statistics. Different types (colors) of mutagens grant different kinds of bonuses.

Each Ability placed in a slot near a mutagen of the same color increases that mutagen's bonus by 100%. So if all three Abilities in a set of slots match their nearby mutagen's color, the mutagen's bonus will be increased by 300%.

Abilities whose color does not match their nearby mutagen have no effect on the mutagen's bonus — they neither increase nor decrease it.