Port 3CF, 07  -  Color Don't Care Register

        1111 xxxx  Reserved
        xxxx 1xxx  Map 3 is Don't Care
        xxxx x1xx  Map 2 is Don't Care
        xxxx xx1x  Map 1 is Don't Care
        xxxx xxx1  Map 0 is Don't Care

        Bits 3-0:  These bits select whether a map is going to participate
                   in the color compare cycle. When the bit is set to 1,
                   the bits in that map are compared.