When you need to quantify the color of ... an off-the-shelf RGB sensor and wire it up to an Arduino. Or you can go back to basics and build this reflective RGB sensor from an LED and a photocell.

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