Don

At the recent 2021 NMRA online convention, Frank Schneider gave a clinic titled " Make Your Own Decals".  This was based on a fairly new technology developed by the folks at http://decalprofx.com/, who have a way to make white decals, as well as a wide range of colors, from black laser printer output.  It's a moderately complex multi-step lamination/delamination based process, and requires some moderately expensive hardware and materials, but should be well worth investigating.

John Stutz