during your plants early stages of growth "veg" they like a blue spectrum of light.metal halid lamps give this spectrum .during your flowering cycle "12/12" your plants want red/orange'ish color,it signifies the end of summer and its time to start producing.hps lamps will give u this spectrum of light.now the newer ballast's u can flip a switch and change the bulb with out having to buy a 2 seperate lighting systemsso basically u get 2 lighting systems in one.personally i just use conversion bulbs but i also run much larger lighting systems than a 150 or 250 watt .