The present invention relates to a hair colouring agent that can be used for cosmetic hair dyeing for dyeing mainly black hair to a bright colour. More particularly, the invention relates to a hair colouring agent capable of achieving cosmetic hair dyeing without using an oxidative dye, and retaining colour for a long period of time. The colouring agent of the invention is prepared by mixing a first agent and a second agent, wherein the first agent contains a dye component containing a basic dye as a main component, and an alkaline agent, the second agent contains a hydrogen peroxide solution as a main component thereof, and at least one of the first agent and the second agent further contains gold nanoparticles or platinum nanoparticles.
