How do I use an LED as a photodetector?
I'm not sure what you mean by wavelength-sensitive. But you can use the LED to respond to light, like a photodiode, albeit a rather inefficient one, which means that it generates only a small current.
Cathode to Vcc, anode via a resistor to ground. If light falls on the LED it will create a current that results in a voltage over the resistor. Like I said, it's not very efficient, so you need a rather large resistance, 1~10 MOhm.
Keep in mind that maximum allowable reverse voltage for LEDs is limited, usually around 5V.