Port 3C0, 12  -  Color Plane Enable

                1111 xxxx  Reserved
                xxxx 1xxx  Enable Color plane 3
                xxxx x1xx  Enable Color plane 2
                xxxx xx1x  Enable Color plane 1
                xxxx xxx1  Enable Color plane 0

                Bits 3-0:  Setting a bit to 1, enables the corresponding
                           display-memory plane.