Marking blue is made by mixing methylated spirits with shellac and gentian violet. This is not to be confused with engineer's blue, made by mixing Prussian Blue with oil.

Marking blue, layout stain or Prussian blue is a dye used in metalworking to aid in marking out rough parts for further machining. It is sometimes called Dykem (after a popular brand). It is used to stain or paint a metal object with a very thin layer of dye that can be scratched off using a scriber or other sharp instrument to reveal a bright, yet very narrow line in the metal underneath. The advantages are that any existing scratches are covered with the dye and the new lines have a contrasting background.