From your symptoms it seems likely that you have tinea versicolor, in which case you would require antifungal tablet and a topical antifungal cream to be applied over the area. Measures that can be used to prevent the condition are: - Avoid using oily skin products - Reduce your exposure to the sun. Exposure to sun may trigger or worsen an episode and a tan makes the rash more visible. - Do not wear tight clothing. - Change undergarments regularly - Wear breathable fabrics such as cotton to decrease the sweating. - Keep areas where there is sweat accumulation dry regularly and apply powder in those areas to reduce the moisture. If the problem is persistent I would advise you to meet a dermatologist.