I know that the list of colors displayed by list-colors-display on X
is fixed (see my message earlier in this thread), but I always
thought, I don't know why, that the length of that list is near the
number returned by display-color-cells.
If this is not true, one may ask what is so ``interesting'' about the
specific colors we show as opposed to those we don't. For example,
when I work on Irix, I generally like to use the Irix-specific colors
(that are not shown by list-colors-display, of course) because they
are much more pleasant to my eyes. So to me, those unshown colors are
much more ``interesting'' than those we show, in that specific case.