A colourant is a substance you add to change the colour of something — most often someone’s hair. Hairdressers put a colourant on hair to make it darker, lighter, or a new shade. The same kind of word can also name a dye in food, plastic, or cloth, though in daily British English food colouring is the more common phrase for what goes in cakes. This is the British spelling; American English usually writes colorant. Though actually, it is a slightly technical, product-label word. At home people more often say dye (for hair or cloth) or food colouring (for cooking). You will meet colourant on bottles, in science class, and in industry.

染色劑（頭髮多說「染髮劑」；食品多說「色素」）