Kolhapuris get darker with time and use. In case of fungus (happens due to moisture and is normal. Don't panic!), clean the chappals with a dry cloth, spray a little with water if required. Apply groundnut/coconut oil periodically to keep them looking new for longer. You can also polish them. Dry them in shade after applying oil or polish. You might find the chappals slippery initially or never. Some people prefer adding rubber sole to them from local cobbler.