did someone already suggest zit cream and sunlight? I've lightened cancer with zit cream, sunlight and time = works best on white ponies as you have to watch real careful for colors to not get a white spotted pony
a bleach bath does good to whiten... also oxy, toilet scrub, ajax kind of cleaner (on body - just watch out on hair as can lighten and don't boil a neon yellow hair pony or you get hair bleed