I have only grown with cool whites. And from everything I have read it is the preferred method. You can grow with warms if you want to though. Its kind of like you can grow veg under a HPS just fine but it is recommended you use a Metal Halide. Or you can grow awesome buds under a metal halide but an HPS is preferred because the red spectrum of light is better for flowering. Also, an HPS 400 Watt will put out more lumens than a MH 400 Watt.