To complete your system pick the bulb that best suits your needs. Metal halide lamps are blue-orientated in the light spectrum and are best used as a primary light source if little or no natural light is available. This type of lamp promotes plant growth. Need a little extra? Pick one of the blue burning high kelvin metal halide lamps for that little extra blue light. If you need a MH lamp for budding or flowering pick one of the high kelvin red burning lamps.