You can throw a Raspberry Pi camera and OpenCV at the problem and approach it through software, or you can buy an off-the-shelf RGB sensor and wire it up to an Arduino.

The Arduino-compatible lighting RGB LED lighting shield reviewed here was designed to give designers a low-cost easy-to-use open-source platform for fast prototyping and inexpensive evaluation of ...

Off the shelf RGB colour pickers allowing you to extract and objects RGB values can range from a few hundred dollars to thousands depending on your needs.