Single process colour is a permanent or semi-permanent hair colour applied in one step, usually an all-over base colour worked from root to tip or a root regrowth touch-up. Unlike highlights or balayage, it produces an even, uniform tone rather than dimensional lightness. A colourist mixes a tint with a developer (peroxide) to open the hair's cuticle and deposit or lift colour, so it can cover grey, deepen or refresh your natural shade, or shift your tone warmer or cooler.