If you're relying on LEDs being monochromatic, you might be surprised to view their light reflected from the base of a CD at the angle that gives the "rainbow" effect. I think it's hard to estimate how much green is in the light this way, but it often reveals the presence of it.
With an incandescent lamp wired with a dimmer, an advantage of dimming is that it shifts the colour of the light towards red. With LEDs, I have read that lowering the current they are passing does not alter the basic colour, but it narrows the spectrum, and this might be helpful too. I can't verify this, I read it somewhere.