Greetings. what are the rashes you are talking about - pimples ?? Pimples may often leave spots especially if not treated or if we burst pimples or pick them. You'll need skin lightening creams for a specified duration to reduce the marks , and this could also be dealt with chemical peeling. Also you must use a sunscreen to prevent darkening of the spots. For scars, creams may not work n you may need procedures like microneedling, microneedling rf, fractional lasers, prp or subscision or a combination of these. Send in your pictures if you need further assistance. Or see a dermatologist near you and discuss the same with him. Hope this helps. Warm regards.