A device such as a computer stylus may have a color sensor. The color sensor may have a plurality of photodetectors each of which measures light for a different respective color channel. The color sensor may also have one or more light-emitting devices. Control circuitry may use the light-emitting devices to illuminate an external object while using the photodetectors to measure reflected light to determine the color of the external object. The electronic device may have a housing with an elongated shaft. The shaft may have a tip and an opposing end. The tip may be configured to emit electromagnetic signals that are detected by a touch sensor in a touch sensitive display. The color sensor may be located at the end opposite the tip, may be located at the tip, or may be optically coupled to the tip using a light guide.