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.