Dye is a substance that's used to change the color of whatever it's applied to. You can use hair dye to change your light brown hair to deep, dark purple.
Dye is used on fabrics, hair, yarn and other fibers, clothing, and food, among other things. When you apply dye to something, you dye it. If you're baking a birthday cake for your friend who loves the color orange, you might use food coloring — a type of dye — to dye the vanilla frosting orange. The Old English root of dye is deah, "a color or hue," which is possibly related to deagol, "secret, hidden, or dark."