while not necessary, the color can produce a better image. I had been using black only negatives from my Epson 7600 for a while. There was always a problem with graininess in smooth midtones (pt/pd prints). I calibrated based on Mark's PDN method, and ended up with a green negative, and the graininess disappeared. Tonal range was still identical... between the two methods.. but i feel i get a better quality images and smoother tones with PDN