So after experimenting with using LEDs as a near space photometer for several years, NearSys is now experimenting with using them in a robotics application. In this case, the unit contains two different color LEDs for detecting and two fo the same color LEDs for a light source.
A two LED color detector being tested on red-colored floor
A robot controller using this unit measures the voltage produced by both LEDs to determine the color being illiminated. The best comparison seems to be made by multiplying the two values. A red and green seems to be most effective at distinguishing between colors as the graph below illustrates.
It also appears that two infrared LEDs, one 870 nm and the other 940 nm are effective at distinguishing between gloss black and flat black surfaces.
Look for a short article about this new color detector and a YouTube video in the near future.
Here's a reference for Mr. Mims' original article
Mims III, Forrest M. "Sun Photometer with Light-emitting diodes as spectrally selective detectors" Applied Optics. Vol. 31, No. 33. November 20, 1992.