Color plane enable     index 12h  ATTRIB

Layout  b5-b4  VSM  (EGA only)
        b3-b0  CPE  Color plane enable

- Color plane enable (bits 3-0)
        The display planes may be selected or deselected depending on
        the values in this field.

        A zero in any of these bit positions will mask the data from
        that color plane. The effect on the display will be the same
        as if that color plane were cleared to all zeros.

        Deselected planes can still be written to or read from in a
        normal fashion. This Color plane enable field affects only the
        output path between the display memory and the color palette.