Quote Originally Posted by thepaan
More or less, yes.

As khyberkitsune said, the spectral content of light leaving the sun doesn't change; only the portion making it through the atmosphere does. I never considered if that would affect growth since I have never seen any studies along those lines. If I had to guess, I'd say it doesn't matter but honestly, I'm not sure.

Back to the question. I don't know why so many people think they need to change the color of the light, though I suppose it works since so many people do it. This is my guess: The cause of flowering (from light) clearly has nothing to do with blue wavelengths because phytochrome simply does not react to blue light. We are left with red light as the causal factor. Since cannabis is a short-day plant (flowering when the days become short or, long-night) and the day length is determined by duration of red light, then exclusive blue light (suggested for vegging) would cause the plant to reach the flowering stage more quickly due to a phytochrome-perceived perpetual night (is this the desired result?). But even MH should have enough red light to cause the day to be perceived and thus to postpone flowering. This is why I can't figure out why people say to use blue light. You can keep the plant from flowering by just leaving the lights on longer. The only advantage I can think of would be to give the plant more light but make it think it has a short day but that is not what they are doing.
The hormone that triggers flowering in cannabis has to build up - ANY light that triggers photosynthesis will destroy this hormone and interfere with the flowering process up until nearly the end of flowering (some clever people run their plants indoors for the first 6 weeks then finish them up outdoors in the summer sun because the hormone has built up so much that the plant cannot easily revert back into a vegetative stage.) Doesn't matter if it's blue light or red.