(Alpha and Integrity servers) PFN_COLOR_COUNT specifies the
    number of buckets (colors) into which all members of the zeroed
    page list and all unencumbered members of the free page list are
    sorted. OpenVMS Alpha systems might derive a preferred page color
    from a request to map a given virtual page and attempt to map
    that virtual page to a PFN of matching "color." This results in
    less variance in which cache blocks are used when accessing that
    page. This might or might not improve performance, depending on
    the application.

    This special parameter is used by HP and is subject to change.
    Do not change this parameter unless HP recommends that you do so.
    If you increase this parameter, you must also increase the ZERO_
    LIST_HI system parameter.