I've got a question: is it possible to add new blood colours that are visible in NPC creation menu ?

Impact Material Types are hard coded -> but you can create custom bloods for Body Part Data (when the creature gets decapped or limb exploded) and Creature Impact Data the grafics that show up when they attack another actor -> the grafics that show up when you attack them (Impact Material Types) are hard coded because they are tied into the Impact Data Sets available surfaces.