I sandblasted the yellow off a Classic II, Mac Plus and Q700. Works better then sandpaper. It leaves the surface a little rough but surprisingly you can actually smooth it out by just rubbing it with your hand. Only downside is you loose the lettering. It would be possible to re-letter it. I've seen it done on other hacked macs. Of course not everyone has an air compressor and sandblaster but maybe someone you know has one. I used silca sand which is very fine. The whole job takes about 30 minutes. Higher air pressure (100PSI) worked best.