Try not to get into any multi-color extravaganzas. Most banner signs are glanced at when a car speeds by or when people stride down the street on a mission. Banners are not meant to be subtle or tricky. In short, don't get artistic. Slap, dab, simple, clean, and done.
Art takes time and costs money.
Polyethylene banners are smooth and slick. Paint selection is important. A coating product for the banner is not cost-effective. Water-based enamels won't hold up in the outdoors. Some banner makers use lettering enamel thinner with acrylic enamel reducer.