I found a nice research paper on using LEDs as wavelength specific light sensors. You can find it on google:

'Spectral and emission characteristics of LED and its application to LED-based sun-photometry'

As one might suspect, the maximum absorption wavelength is blue-shifted compared to the emission spectrum. So whats nice is that LEDs are dirt cheap, and the yellow ones are sensitive to Green light, but reject blue and UV. They might make an inexpensive sensor for my diagnostic device. I have ordered a bunch, from multiple manufacturers, and well as regular RGB detector devices. Let hope I find a cheap solution!