ADA Signs are commonly made of plastics, but that is not the only choice. Do you know what is available? There are many materials, but you should know this 3 substrate materials and a little about the colors and materials used for rasters (the bumps used in Braille).
Plastics: Companies like Gravograph, Gemini and RowMark make full lines of plastics for the substrate and the applique. They are readily available and easy to work with. The finish is the correct matte surface and the colors contrast well. Using these families of plastics is common and very convenient.