Adafruit RGB Colour Sensor - TCS34725
Your electronics can now see in dazzling colour with this lovely colour light sensor. Adafruit found the best colour sensor on the market, the TCS34725, which has RGB and Clear light sensing elements. An IR blocking filter, integrated on-chip and localised to the colour sensing photodiodes, minimizes the IR spectral component of the incoming light and allows colour measurements to be made accurately. The filter means you'll get much truer colour than most sensors, since humans don't see IR. The sensor also has an incredible 3,800,000:1 dynamic range with adjustable integration time and gain so it is suited for use behind darkened glass.
The board includes supporting circuitry as well, such as a 3.3V regulator so you can power the breakout with 3-5VDC safely and level shifting for the I2C pins so they can be used with 3.3V or 5V logic. Finally, Adafruit specified a nice neutral 4150°K temperature LED with a MOSFET driver onboard to illuminate what you're trying to sense. The LED can be easily turned on or off by any logic level output.
Connect to any microcontroller with I2C and Adafruit's example code will quickly get you going with 4 channel readings. Adafruit include some example code to detect light lux and temperature that they snagged from the eval board software.
A detailed tutorial is available from Adafruit, check out the Arduino library and follow their tutorial to install. Wire up the sensor by connecting VDD to 3-5VDC, Ground to common ground, SCL to I2C Clock and SDA to I2C Data on your Arduino. Restart the IDE and select the example sketch and start putting all your favorite fruit next to the sensor element!