Couple tests of white acrylic with black acrylic paint filled into etch. Followed by sanding down the entire face. In this case these tests had the protective paper etched through. I simply left it on and sanded it off along with the extra paint.
-Heavy raster used in file.
-black acrylic painted onto etched face and allowed to dry; entire face as then sanded until the etched image was revealed. The result is a dull white for the plastic that reminds me of bone somewhat.
-Overcoat of flat sealer was used. Perhaps a gloss coat would return some of the shine that is loss during sanding.
The result is I can't remove the paint with a finger nail and rubbing a moist paper towel doesn't cause the color to come off.
Note: One has to be careful sanding as the etch is not very deep and you can easily sand the image away with rougher grit sand paper. Having an outline or ridge on the outer edge of the image helps prevent loss of etchings due to over sanding and in general protects the ink better. See the skull with swords for example of sanding out the etch.