I think I've used a similar, home made version of this in the past. I essentially mixed P3 inks, with vallejo glaze medium and a tiny drop of paint, to produce this very fluid wash, that went into recesess, created a smooth transition and did not affect the raised areas as much as GW shades/glazes do. It was very difficult to mix though as it often came out too glossy, or too thin etc. Gonna try this out, have something like 70 skeleton warriors that I would love to do without much hassle lol.