(Growth of mold/fungi especially in humid and warm climates). Black, gray or brown spots or areas on the surface of paint or caulk. Mildew is not caused by paint, it is caused by fungi feeding on oil contained in paints or on thickeners in latex coatings. Fungi also feed on nutrients underneath the paint film. Lengthy periods of warm, moist conditions can activate spores on what appears to be a clean surface.