Another interesting tidbit in the paper referenced by Terry that caught my attention - green light isn't useless! It penetrates deeper into the leaf than blue light. Another reason not to use the hideous red-blue LEDs.
Yes, the plant uses the entire photosynthetic spectrum, but the areas around red and blue are absorbed the most efficiently. If you don't care about appearance while growing, are trying to maximize the light generation per watt used - maybe so the police can't use your electric bill to pinpoint you - it's still a good way to go.
However, I agree with you about the hideous appearance. Since the human eye is most sensitive to the green part of the spectrum, it seems to me that adding enough to please the eye and feed the right amount of photons to the plant ought not be THAT inefficient.
Ray Barkalow
firstrays.com