Found this on a German hobbyshop.

https://www.dersockelshop.de/grafx-art-masking-liquid-pen-4mm_molotow_728002.html

Sound like a good idea for masking camouflages.

I was intrigued by those myself when I saw them in the newsletter today . There are a few videos on Youtube showing how to use them and the Molotow products seem to be a thing in the crafting world.

From what you can see in the videos, these behave pretty much like normal markers do with a light blue fluid which can then be rubbed off apparently easily. Most examples are using these pens on plain paper in combination with water colors so not sure how well it will coop with acrylics or enamels.

I'd certainly be interested in case someone gives this a try!