As explained by Arduino to Go, each color band on the resistor corresponds to a number but this can differ depending on the band's placement. It's not complicated, but the sheer number of colors ...

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.

This minimalist beauty uses an Arduino Nano and RGB sensor to assess the color. At the top, a small servo rotates an arm inside the hopper that both shakes the Skittles and sets them up single ...

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.