The information below is taken from the RasMol Reference Manual at http://www.umass.edu/microbio/rasmol/distrib/rasman.htm.

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 colours `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.
(Information below is based on RasMol's source code and as of February 19, 1998, is believed to be complete and correct here for the first time.)

    ELEMENT                   COLOR NAME       RGB DECIMAL    RGB HEXADECIMAL
    Carbon                    light grey       [200,200,200]  C8 C8 C8
    Oxygen                    red              [240,0,0]      F0 00 00
    Hydrogen                  white            [255,255,255]  FF FF FF

    Nitrogen                  light blue       [143,143,255]  8F 8F FF
    Sulphur                   sulphur yellow   [255,200,50]   FF C8 32
    Chlorine, Boron           green            [0,255,0]      00 FF 00

    Phosphorus, Iron, Barium  orange           [255,165,0]    FF A5 00
    Sodium                    blue             [0,0,255]      00 00 FF
    Magnesium                 forest green     [34,139,34]    22 8B 22
    Zn, Cu, Ni, Br            brown            [165,42,42]    A5 2A 2A

    Ca, Mn, Al, Ti, Cr, Ag    dark grey        [128,128,144]  80 80 90
    F, Si, Au                 goldenrod        [218,165,32]   DA A5 20
    Iodine                    purple           [160,32.240]   A0 20 F0

    Lithium                   firebrick        [178,34,34]    B2 22 22
    Helium                    pink             [255,192,203]  FF C0 CB
    Unknown                   deep pink        [255,20,147]   FF 14 93