Discolor is the American spelling of discolour. The meaning does not change: a thing discolors when its colour shifts in a way that looks worse — a white shirt gone yellow, a cut apple gone brown, a wall the sun has stained. American books, sites, and product labels write discolor; many Taiwan textbooks write discolour. Neither is a different word. They are the same verb in two jackets. Though actually, you will meet both in the same week online, so it helps to treat them as twins, not as a right spelling and a wrong one. The matching American noun is discoloration.

（美式拼法）變色、褪色。台灣課本較常見 discolour。