To get a handle on this I bought some oil paints: black, umber and red for starters and some turpentine-replacement. The good thing about those is that when applied on clear varnish, they can be pushed around for a really long time compared to acrylic paints and shade. It also flows really nicely along ridges and into recesses.
The guadians I was working on got hit with clear varnish (actually Emsal Voll Glanz, a Johnson's Klear equivalent floor varnish). 24 hours later I got the oil paint mixed up, loaded up a brush I marked for use with oil paint and got going.
The picture below shoes how nicely the oil shade runs into the recesses outlining the guardian's sixpack.
A few days later I varnished the wraithguards and applied oil paint to them as well. For the picture below I grouped them all up and left one model from each group without the shade. These are marked with an X. The fact that they blend in quite well implies that the shade isn't a big addition, but when looking at the model up close, the shade really makes it look much more substantial.
I now varnished a wraith loard some war walkers and a bunch of rangers and will go for them next. Then I need to determine what I will finish this process with. Another coat of varnish, possibly matt or satin most likely.