You can get colored acrylic or cellulose (plastic) from hobby supply stores.

Perhaps you have one of those 3D glasses from the movie theater (red and blue) ?

Stick the filter in front of the optical detector, the led or both (make sure its not reflecting the light directly back into the detector).

Might have to adjust the gain, threshold etc of the sensor to get the right behavior.

Some leds and detectors prefer different light wavelengths and that's a better way to go, but more difficult and costly.

^{_{So long, and thanks for all the fish.}}