Sometimes even within symbolic icons, parts of the design must keep
fixed colors, when they are semantic. For instance this is the case for
the color-picker-(black|gray|white) icons or the gimp-default-colors.

The trick used by GTK+ to recolor is by supplying an additional CSS
style to icons which overrides inline CSS thanks to "! important"
keyword. Our own trick to circumvent this trick is to set "!important"
in inline CSS when one wishes not to have it overrided by the general
colors.
Right now I am doing the edit by hand with vim because Inkscape doesn't
seem to validate this keyword if I add it manually in its XML editor. So
what I did is add the "color-important" label on such object, and I will
later modify my SVG export tool to add !important on such object "fill"
values at export time.