I just took a few of those stupid colored flames off of a Kenda with carb cleaner and 0000 steel wool. Had another set and the white was raised and it was actually a big chunk of white rubber made into the tire....Non removable. If you can't get it off some people have used black spray paint and just cover them up.
You might try Goo-Gone Extreme ($4.00 at Walmart). It is a little less harsh than acetone. I have also had them come off with Simple green and a scotchbrite pad. Just do a little test spot before going full bore.