What makes it confusing is Prussian blue is basically a color. Its like saying crimson or purple. The use of the minerals in pigments & inks go back a long, long time. You'll see it named in any artist paints color set. Apparently
the name Prussian blue originated in the 18th century, when the compound was used to dye the uniform coats for the Prussian army. Over the years, the pigment acquired several other “blue” names, including Berlin, Parisian, and Turnbull's blue.
en.wikipedia.org
Maybe a better way to classify is 'layout' or 'transfer'. Layout means an opaque, quick dry coating on the metal to allow you to scribe very thin but distinguishable lines via the shiny contrasting metal color. The typical stuff is alcohol based, so dries relatively quick & (mostly) withstands cutting oils. Comes off readily with methanol. You can also get in aerosol for larger areas. You can also get red or a few other colors Sharpies serve the same purpose, handier & faster drying for smaller areas.
STARRETT,STARRETT 4oz KLEENSCRIBE LAYOUT DYE,1-855-53212,KBC Tools & Machinery
www.kbctools.ca
Transfer is also known as 'high-spot'. Typically its a greasy oil based cream viscosity, but now there are water based & other formulations. I don't think it really dries, or if it does, over a very long time. Its used in scraping where you roll it on a surface, for example the granite surface plate, rub a part on it. What transfers off to the part are the highs or hilltops to be scraped. Or similarly any 2 surfaces where one is considered a master, say a bearing & shaft assemblies. Sharpies also work here, but generally smaller parts. But you don't use transfer medium as a layout medium or vise-versa.