Prime in Chaos Black , then Runelord Brass . You can also airbrush this.
Fill in the none-metal sections with Abaddon Black or AK Black (or any black of your choice!)
Paint the metal parts Leadbelcher or Two Thin Coats equivalent (I prefer the coverage of this). I leave part of the barrel Leadbelcher.
Wash the Brass areas with a thinned Guilliman Flesh (2:1 Contrast Medium ), which you pull towards the recesses.
Line shade the deepest recesses with Cryptek Armourshade Gloss . I also glaze some of this away from the recess part way up the previous shade.
Cover the silver metal areas with Nuln Oil
Glaze some of the Runelord Brass back on the panels.
Edge highlight with Canoptek Alloy . I also do a small glaze of this, leaving a large amount of the Runelord Brass beneath.
Dot highlight with Silver RLM01 Metallic . On vehicles, this would also be an appropriate sponge colour to stipple damage.
On any shoulder pad/knee, base with Mephiston Red . Line shade recesses with Nuln Oil , then glaze Flesh Tearers Red around recesses. Tidy back with Mephiston Red .
Edge highlight Evil Sunz Scarlet and final dots and scratches of Wild Rider Red . Glaze the whole lot with a thinned Baal Red to bump the red back up.
I paint the gun casing Nuln Oil Gloss , then paint chips with Silver RLM01 Metallic .
Eyes, poison pipes, weapon poison etc
Basecoat White Scar or AK White (my preference)
Apply a couple of layers of Greenstuff World Fluoro Green, then recess with Hexwraith Flame . You could use any Fluoro green, or use Tesseract Glow
Chunky highlight of Incubi Darkness
Thin highlight of Thunderhawk Blue
Dot highlight of Russ Grey . Glaze with Terradon Turquoise thinned heavily with Contrast Medium . On characters, you can go as far as Fenrisian Grey and then Blue Horror on very highest/brightest points.
Any bones, basecoat Grey Seer
Shade with Basilicanum Grey or Apothecary White . Deep shade Nuln Oil .
Highlight Grey Seer
Highlight White Scar
Base with Cadian Fleshtone , wash Guilliman Flesh , highlight glaze with Cadian Fleshtone , edge with Kislev Flesh then add dots of Ice Yellow or a bone colour. You can also highlight using Ionrach Skin then Deepkin Flesh to push a colder look, highlighting that further with white.