From the Super_DJC Driver docs (commercial driver)

The Hewlett-Packard yellow-magenta-cyan (YMC) inks combine to produce a
dark olive green instead of a true black.  HP recommends decreasing the
cyan component on a color by color basis, depending on the "grayness" of
the color.  This is defined as the maximum intensity minus the maximum
difference (max - min) of YMC intensity values.

   Other drivers, to speed things up, simply decrease cyan across the
board, regardless of grayness.  Super_DJC, on the contrary, follows the HP
recommendations, and in addition allows the user to choose the fraction by
which the cyan component is reduced in a color, as a function of its
grayness.  That fraction is calculated as (Threshold - 2) / 18, ranging
from 0 to 13/18.  HP recommends reducing cyan by 1/3, which corresponds to
a Threshold value of 8.

