I have been given a corporate logo to use as the basis of an icon.  Its
colors do not match the ones in the resource editor's palette for 4 and 8
bit icons however I have been able to import my own palette.  Suppose my
application is run on a host that only supports 256 colors.  I understand
from various other posts that the host will probably choose my 4 bit icon so
that its colors and colors of other screen elements can all be
simultaneously displayed.  But suppose also that my corporate logo colors
are unusual enough that more than 256 colors are needed to display
everything on the screen.  What happens?  Also, what happens in a very old
system that only supports 16 colors?

This document
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwxp/html/winx
picons.asp
shows 31 "primary colors" that are used in Windows icons but nothing more is
explained.  Is the implication that only colors in this set of 31 should be
used to minimize the color variability across the universe of icons which
might be a particularly good thing if there are only 256 system colors?