Hyperpigmentation is the scientific term for red or dark marks that are left behind after acne lesions heal. These marks are much more common in people with non-Caucasian skin.
Prevention: The best treatment for hyperpigmentation is prevention. Clear your acne using a proper skincare regimen to prevent hyperpigmentation in the first place.
Treatment: For marks that are already existing, rule #1 is to keep them out of the sun and wear sunscreen. This will help them fade faster. Then, some hyperpigmentation treatments are available, such as glycolic acid, which can keep the skin turning over and speed the healing of these marks. There are many other topical treatments that can help to some degree as well. However, don't look for miracles. Hyperpigmentation can be stubborn. It bears mentioning one more time that the best treatment is prevention.