Engineers blue

[Claudio HG]

You can make it using (the blue fluid)
400ml alcohol
18~20ml aniline paint (or you can use any anilline paint color)
50~60 ml shellac
and a good shake
until thin all shellac.

This is not what the OP was looking for. Engineers' blue is the color used to enhance high spots when scraping flat parts. It must not dry fast, it must be easy to clean up, it must be easy to see, it must be very fine to spread a very thin layer on the surface plate. This is accomplished by Prussian Blue or any color of your preference that is based upon linsed oil, essentially the colors used by artists for oil paintings.
I use ultradeep blue and red magenta because I see it better. It drys in 24 hours (less in hot climate) or just never dry out if the color is home made using dry pigments and mineral oil (in minute amount). And it can be easily cleaned up using mineral oil and then (after the color is removed with paper towels) >90% ethyl alcohol (just spraying the surface to clean up).