Power wash the tiles with a trisodium phosphate (TSP) cleaning solution, using a garden sprayer. The TSP will remove dirt and debris from the tiles. Use a stiff brush to dislodge any stubborn dirt around the tiles and rinse the roof with plain water, using a garden sprayer. Allow the roof to dry overnight before proceeding.
Lay drop cloths on the ground around the roof. Cover anything you don't want to get paint on with painter's tape and newspaper.
Prime the roof with masonry primer, using a nylon paintbrush for textured roofs and tight spots and a foam roller for large flat areas. Alternately, use a paint sprayer to distribute an even coat of primer over the tiles. Allow the primer to dry overnight.
Apply a second coat of primer using the same techniques used to apply the first coat. Let the primer dry overnight.
Apply a layer of latex paint using the same methods used to apply the primer. Let the paint dry overnight, then apply one or two additional coats.