HiTechnic has finally released its new Color Sensor. It can be purchased directly from HiTechnic or from LEGO.
"The NXT Color Sensor operates by using three different color light emitting diodes (LED) to illuminate the target surface and measures the intensity of each color reflected by the surface. Using the relative intensity of each color reflection, the color sensor calculates a Color Number that is returned to the NXT program.
The Color Sensor connects to an NXT sensor port using a standard NXT wire and uses the digital I2C communications protocol. The Color Number calculated by the sensor is refreshed approximately 100 times per second. "