When you use red and blue LED's together, don't the two spectrums cohere? I believe it creates a bell curve between the blue and red spectrum. It follows the theory of RGB light, you mix them and get white, you mix red and blue, you get a purplish-red (at least with LEDs and their high red to blue count).

The plants do use more than one wavelength, but can they survive on one wavelength? It depends, I hear that once light enters the plant material, it hits Chlorophyll A and then B or something (could be backwards on that one) and that the wavelength of the light could change during that. Also blue and red LED's aren't one wavelength, they are a combination of wavelengths of almost the same color, like red LEDs are 600-670nm wavelengths, that are all in different ratios, with the peak output of wavelength 627nm(on average).

I think you can grow with only one color..blue or red, but the emerson effect is like a car upgrade. Red makes the plant grow, blue makes it grow, but if you combine them like numbers and those numbers are photosynthetic rates...then red is 5 rate and blue can be 5.....so you would think both together would be a grow rate of 10, but it's actually probably a 12-15 grow rate. It's the emerson effect (thanks physicsnole, I believe).