You can use a led as a light sensor and colored leds to sense colored light. Using leds with colored bulbs to filter what gets in, only light of the emitted wavelength or shorter will be read but the bulb filters out the shorter - you can get color band strength. Do for Red, Green and Blue since what you see may be a mix.
It is slower than more costly sensors but faster than human and the cost is very low.
I have my own version of this project. Both work. It is not color-specific.
http://playground.arduino.cc//Learning/LEDSensor
Mitsubishi Labs white paper on using leds as sensors for communication and light level sensor using the same led that emits the light being controlled.
http://www.merl.com/reports/docs/TR2003-35.pdf
Liquid ID Spectrometer.
http://creative-technology.net/MAKE.html
See what your professor says. For less than 1 Euro you can add RGB detection to Arduino.