Black, grey or brown spots or areas on the surface of the paint.
Test for mildew by applying a few drops of household bleach to the area; if it is bleached away, the discoloration is probably mildew. Remove all mildew from the surface by scrubbing with a diluted household bleach solution (1 part bleach to 3 parts water), while wearing rubber gloves and eye protection. Rinse thoroughly. To protect against mildew, use a high quality waterbased paint and clean when necessary with the above bleach solution or neutral detergent. Consider installing an exhaust fan in high moisture areas.
Source: The Paint Quality Institute