Decades ago, I built a sensor that detected a break in a path to a retro-reflector up to 6 feet away using a single bi-color LED and some op amps, etc. I don't remember all the details... but I did learn how the photoelectric effect works, in my gut.
A red LED emits a lower energy photon than a green LED. Each will pick up their own reflections, but the green LED will not pick up the light from the red one.
A red LED emits a lower energy photon than a green LED. Each will pick up their own reflections, but the green LED will not pick up the light from the red one.