I chose Night Lords. I've been reading the Dembsi-Bowden novels and I can't recommend them highly enough. They are a wonderful source of inspiration and a fascinating portrait of a Legion descending into heresy (they admittedly start on some pretty shaky ground). As a First Founding Legion they also give you the opportunity to mix in Heresy plate, and the Night Lords' colourway remains pretty consistent pre and post Heresy. They feel very '40K' with their 80s-heavy-metal aesthetic of skulls, bat wings and lightning. Plus they'll make a nice contrast to my more heraldic, red-or-bled Blood Angels, who are the antagonists in one of the novels.

Full of excitement, my first step is to do lots of visual research. I try to find examples of great paint schemes for that army and look for new twists I can put on the subject. You can see the results below. I also talk to anyone who collects that army, and Mr Will Hayes from Forge World was very gracious to tell me about his beautiful Night Lords collection.
The next step is colour tests. The Citadel push-fit CSMs are great for this, they require very little build time. I'm always keen to try as many techniques as possible at this stage and 'go wide' in my experiments. I'll vary washes, choose different colour mixes and try to cover as many bases as possible (pun intended). I write down which paints I used for future reference. Here are the three resulting miniatures.
One of the challenges with Night Lords is to avoid them becoming too garish. The lightning pattern can be quite distracting so I was keen to keep this subtle. Hence it has become something more akin to a crackle texture rather than very obvious graffiti (besides, some of the lore implies the lightning is not simply a design painted on to their armour, but a reactive substrate that 'crackles'). I experimented with making it more of a graphic pattern too. This worked OK but would need to be knocked-back a lot I feel. I think using it sparingly on infiltrators would be cool. Plus I'm fond of camouflaged tanks for Marines, so if I ever get around to doing a vehicle it could become the machine's camo pattern.
In the end I plumped for the paint recipe of the guy in the middle with a few variations. I feel his red horns are a little too bright.
Next stop - the first 'proper' mini.