look at this patched pixbuf screenshots to understand http://www.opendesktop.org/content/show.php/gtk2-engine-pixbuf%20(Patched)?content=77783
Created attachment 3523 Draw insensetive icons transparent instead of pixelated. The gtk xfce engine uses the default gtk icon render functions. Probebly (most) other theme engines do that too. So this bug could be reported to gtk to get a more general solution. This patch makes transparnt icons posible by wrapping the default pixelbuf renderer. It shouldn't be hard to tweak this patch to get the transparency and saturations values you like the best.