I'm mesmerized by this. Says Reddit commenter dimmsimm:

This process was showcased on an episode of OC Choppers a couple of years ago where Paul Jr. had a bike tank, fenders and other bike parts dipped in a water tank with a floating camo pattern that was either printed on the water or placed on the surface using a sacrificial carrier sheet that dissolved in the liquid. This is a very old process with its origins in "marbling", a technique used for, among other things, putting an interesting, decorative pattern on end papers in case bound books.

Painting on water:

Hydrographic painting:

See also: how to paint a guitar and how to paint a gun