I've been having a think about the rumour that it's a one-coat colour paint, not least as it's starting to pop up from other sources that are reportedly reliable.
It's certainly feasible. A paint that dries darker in the recesses, and lighter on the higher areas? That's pretty much the definition of a wash. This can already be done for a nice effect, with yellow, with multiple thin coats - even a glaze will do this to some extent.
Oil-based dip works this way somewhat too, but goes more into the recesses and grades over the existing colour base.
So what you'd need is something with a much higher pigment amount than a a standard Shade paint, more similar to a proper ink to get that coverage. You'd need to get the surface tension just right - so it will gather in recesses to dry darker, but not so much that it pulls away from the rest of the surface. You'd also have to increase the drying time significantly I think, and keep the pigment nicely in suspension throughout the medium as it dries. With standard washes, that's why you get random patterns and tide marks - the edge of a pool dries first, leaving the infamous bath ring of pigment, and not much left in the rest. This is definitely solvable - you get it for free with oil-based paint - but GW must have contracted out some interesting chemistry to get it to work.
The two closest examples I can think of currently are
1) the dear departed modelmates rust effect. This is water based, goes on like a paint, but dries different colours depending upon how thick it is; a thin area is yellowish, a medium coat reddish, right down to thick, flaky red-brown when layered heavily. It's too thick to naturally gather in recesses on its own, but a thinner version would definitely work as a single coat paint - as long as you want it rust coloured!
1) hexwraith flame somewhat - and even more so, nighthaunt gloom. These are nearly there for what the rumour says in a single coat, without the blotchiness or tide marks of a normal wash - it's an acrylic, but quite clever. Do nighthaunt gloom which shades a little more in a variety of colours, drybrush to boost the highlights and you have a 2-stage process that will give you a pretty decent effect I reckon.
I'm not sure how good such a paint would be on models with a lot of flat surfaces - i.e. space marines - but for anything organic - cloth, skin, hair, leather, I can see it being really quite a time saver. Assuming it is true, It'd be interesting to see if putting it over a black->white zenithal highlighted model helps or hinders the effect!
Here's a screenshot of a single heavy coat of nighthaunt gloom, for example:
Edited by Arkhanist, 03 May 2019 - 05:56 PM.