I think I read somewhere on the internet that an LED could be used as a wavelength-sensitive photodetector. Could any of you point me to a electronics beginner-friendly article or tutorial on how to do this?
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.
I published a quick circuit using LEDs as photocells in SILICONE CHIP in the pre internet days I used cheap standard 5mm red LEDs I started by giving the LEDs various light sources finaly taking them outside in the hot Christchurch sun where the DVM gave me over 1 volt MY DVM had an input impedence of 10 megohm so for the job in hand which was just a one off I lashed a simple transistor circuit with 10 meg between base and emitter which worked fine and got published SO yes they do work and its very cost effective because the LEDs were standard parts bin types BUT they don't specify them as such so large scale manufacture could be risky if any accuracy is needed MY circuit didn't want or need to be fast so the 10 meg base emmiter resister was an asset NOW if you need to be fast a transimpedence amp will work This was proven in production by using a HP infrared fibre optic transmitter as a receiver because the stock receiver drew too much current at 5V