The RasMol cpk colour scheme is based upon the colours of the popular plastic spacefilling models which were developed by Corey, Pauling and later improved by Kultun. This colour scheme colour `atom' objects by the atom (element) type. This is the scheme conventionally used by chemists. The assignment of element type to colours is given below.
Carbon light grey Chlorine green Oxygen red Bromine, Zinc brown Hydogen white Sodium blue Nitrogen light blue Iron purple Sulphur yellow Calcium, Metals dark grey Phosphorous orange Unknown deep pink