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. Or you can go back to ...

Arduino enthusiasts may be interested in a new single pixel scanning camera which is powered by an Arduino Uno combined with a TCS34725 sensor available from online stores such as Adafruit.

[Fjord Carver] brings together an RGB LED and CdS Photoresistor ... This time around it’s an Arduino doing the measuring instead of a PIC. So why isn’t that sensor shown in this picture?

If you’d like to get more specific, Michael Klements’ Arduino-based scanner lets you quantify colors in numerical RGB values via a TCS34725 sensor. User interface for the handheld device is ...