Try one of those little black abrasive sanding blocks, start with a fine and go to a medium if needed. Keep it from rubbing on ther edge of the tile, probably wont do any damage anyway, but rather be safe than sorry. Just work the edge of the block into the grout joint where the paint is. Things like Goof-Off may remove the paint but also discolor the grout. A grout haze remover, sold at tile stores and a stiff toothbrush to agitate the paint may work also.