I suspect that Francesco had the same problem I did. In the color picker
dialog box, there is an icon, about 2/3 of the way from the top, which
when one hovers the mouse cursor over the icon, reveals the context help
that one can select a color from anywhere in the screen. When one's
cursor is located over a GIMP window, this is true, one can select a
color; when the cursor is over a part of the screen that is not part of
a GIMP window, the cursor reverts from the eyedropper shape to the
normal cursor shape, and in my experience, one was not able to select a
color from any part of the screen that was not a GIMP related window.
This is a known gtk+ issue in windows (bug 653653) at bugzilla.gnome.org.
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