Still mucking about with camouflaged Primaris marines, I found this tutorial (EXCELLENT if infrequently updated blog btw, highly recommended nonetheless) and as you can see I decided to give it a go myself.
The colors I chose were:
Celestial Grey Base coat
Mechanicus Standard Grey as the first layer of blotching
Macragge Blue as the second
Daemonette Hide as the third.
After which came detailing, painted solid colored areas, decals, etc.
Then a light inking of Agrax Earthshade
...and lastly a light dry brushing of Bleached Bone.
To be honest I rather like it. Given just how many Primaris marines I have, I might even paint a small army of these guys just for the hell of it! I could even fight side-by-side with Neverness' Smurfs vs. some vile foe, though I'd dread to contemplate just what logistical nightmares and celestial alignments would be required for that to occur...