dis·col·our discolours discolouring discoloured in AM, use discolor If something discolours or if it is discoloured by something else, its original colour changes, so that it looks unattractive. A tooth which has been hit hard may discolour Some oil had seeped out, discolouring the grass. + discoloured dis·col·oured Some of the prints were badly discoloured. + discoloration dis·col·ora·tion the discoloration of the soil from acid spills. to change colour, or to make something change colour, so that it looks unattractive