If you design your icons in SVG with Inkscape and wonder how to quickly make an "inactive" icon in grayscale, here's a quick fix (at least for Linux users):
inkscape -f teacher_na.svg --verb EditSelectAll --verb org.inkscape.color.desaturate.noprefs --verb FileSave --verb FileQuit
This will edit your file in-place (so you need to take a copy first) and will require Inkscape to be launched, but all in all, this will make you save a lot of time.
