As this chart illustrates, chlorophyll activity peaks at 6000K & 5500K which is best provided with the Super MH or Cool MH type lamps, and again at 2700K and 2200K which is best provided by the Super HPS (Son Agro, Hortilux) and the standard off the shelf HPS lamps. As you can see from this PAR chart, standard MH lamps operating around 4000K should be avoided as they have little to offer in the way of Photosynthetically Active Radiation (PAR).