Heritage Preservation’s Rescue Public Murals initiative has been gathering best practices for mural creation. Some basic information is on our Web site at http://www.heritagepreservation.org/RPM/MuralBestPractices.html and we are working on improving this site over the next few months. The site describes some research on exterior paint longevity that is being done at the University of Delaware. Muralists in the field report that the acrylic paint systems made by Golden Artist Colors (www.goldenpaints.com) and Nova Color Paints (http://novacolorpaint.com) do well long-term. Both companies have good customer service that can suggest colors that are durable and historically appropriate for your purposes. Two additional companies that have high quality exterior paint but may have more limited color selecteds are Keim (www.keim.com) and Sherwin Williams’ Sher Cryl marine paint (http://protective.sherwin-williams.com/detail.jsp?A=sku-26028%3Aproduct-6795).
I agree with Helena that you begin the project with a clear layer to seal the original paint. Note that this layer may not be reversible. Ask the paint manufacturers for their advice. Rescue Public Mural used this approach on a restoration project in 2009. See http://www.heritagepreservation.org/RPM/archive6.html.