I wouldn't even attempt to paint HDPE. That stuff doesn't like anything to stick to it. If I were doing golf signs at any kind of scale I'd do this:

http://www.kingplastic.com/products/king-colorcore/

Just mill into the material and expose the brighter color underneath. This stuff sticks to a vacuum like glue, and if you use the right speeds and feeds requires very little, if any cleanup afterwards. Very effecient.