FWIW, there is a graph somewhere that shows the wavelength of maximum sensitivity of the human eye as the intensity of light diminishes. I'll see if I can find it.

Also, the output spectrum of LEDs is wider than most would imagine. Probably not an important issue for this purpose, but a yellow filter might help cut some blue out of the green LED light. I assume that the blue would be better removed.

View the light of your LED reflected from the surface of a CD at the angle that shows the "rainbow" effect and you might be surprised to see what's there.