To discolor means to change color in an unwanted or unattractive way, usually by staining, fading, or turning dull. When something discolors, it loses its original color and becomes less appealing.

White shirts often discolor to yellowish or grayish shades after many washes. An old book's pages might discolor from bright white to brown or tan over the years. If you spill grape juice on carpet, it will likely discolor the fibers, leaving a purple stain. Teeth can discolor from coffee or tea, shifting from white to yellowish-brown. Even metals discolor: silver tarnishes to black, and copper develops a green coating called patina.

The word suggests damage or deterioration rather than an intentional color change. You wouldn't say that dyeing a white shirt blue discolors it, because that's a purposeful transformation. But sunlight slowly discoloring that same shirt to a washed-out pale blue? That's unwanted change.

Scientists use the word discoloration (the noun form) to describe these color changes. A doctor might notice skin discoloration as a symptom of illness. A conservator working on an old painting watches for discoloration as a clue that the artwork needs special care to preserve it.