An array of RGB LEDs shine light down onto the surface of the track. The intensity of the reflected light of each spectrum is measured by the voltage value across an array of phototransistors aimed at the track. The values are converted to digital values that the microcontroller can process by using the PIC18F46K20's onboard ADC.

 

The phototransistor was chosen after several tests with different sensing components. Components tested include phototransistors, photoresistors and photodiodes. The video below shows a test done with a photoresistor.