. During its vegetative phase, cannabis appears green (and grows, well....like a weed) because the predominant pigments (chlorophyll a and b) are geared more towards absorbing higher-energy (shorter wavelength or higher "temperature") blue and violet light, which is abundantly present in the white light from the summer sun. The light that is reflected is only the portion of white light that isn't absorbed by the plant (the same concept applies to anything with color - we only see the wavelength(s) of light reflected into our eyes). This is the first step of photosynthesis - capturing the sun's energy through light-responsive pigments. As the plant reaches the end of its life (generally during the fall), chlorophyll degrades and the remaining pigments become predominant and more visible (hence the yellow/red/orange leaves on trees in the fall). These pigments absorb lower-wavelength (more "reddish") light. This is why most growers will use "bluer" metal halide lamps during veg then switch to "redder" sodium lamps or similar for flowering. This doesn't mean that an entire crop can't be successful with just one type of lamp - heck, my small hobbyist setup runs off one HPS lamp and does fairly well. Just make sure your lamps are the proper wattage for the area you're using (otherwise the growth directly under the canopy will remain too shaded).
