Shoudl not be too hard/
Buy an i2c light & Color Sensor like this for example:
http://www.ebay.com/itm/Light-Color-Sen ... 0906330001
Just make sure to get a module that uses 3.3v and uses i2c communication.
You can drive it directly from the gpio pins as requires has 3.3 volt input.
Buy a shield for the Pi and solder the component on that. Make the correct connections.
If you want to program in Python, try to find a module for it. If not see if someone has demo code, or use the datasheet. Do you have experience with programming the gpio and i2c?