Are your sneakers, flat slippers or espadrilles stained...or just tired-looking?
Tie-dye them in variegated shades. No dipping, either. WAYYY too easy...which makes it even more interesting!
I could see doing this with anything textile-related, but would double-check for colorfastness. How do you do that? One easy way: wet a q-tip, then rub it against the item. If color comes off, you need to set that dye. (Heat-setting it with a hot iron works, particularly if the item in question is still damp.)
Instructions here, thanks to Instructables, a great site for all sorts of how-tos.
Another version, with a more 'marbled effect,' using shaving cream.
(No, I am not making this up.)