This is my take on Chapter Master Valrak's scheme.

The main colours are silver, gold, and red.

General Information:

My general method is to lay down all the base colours for the model, then do all the shading in the recesses before moving on to the highlights.

I don't own an airbrush, so the recipe will mainly consist of brush on paints.

Priming:

I find the metallic sprays from GW tend to be a little on the transparent side, so an initial undercoat of grey helps.

Because the shoulder pads, backpack, and helmet are mostly silver, I mounted those parts on paperclips to make the painting process a little easier.

1. Prime all the parts lightly with Mechanicus Standard Grey
2. Prime the head, shoulder pads, and backpack with Leadbelcher
3. Prime the main body with Retributor Armour

Gold (Armour Panels - Body, Arms, Legs):

1. Drybrush over with Retributor Armour to even out the coverage
2. Shade the recesses with Agrax Earthshade
3. Add a thinned layer of Rhinox Hide in the deepest recesses
4. Highlight fine points and sharp edges with Stormhost Silver

Silver (Armour Panels - Helmet, Shoulder pads, Backpack):

For grenades, replace Nuln Oil with Athonian Camoshade

1. Drybrush with Leadbelcher to even out the coverage
2. Shade the recesses with Nuln Oil
3. Roughly highlight the edges with Ironbreaker to create wear and scratches
4. Add a fine highlight of Stormhost Silver to the sharpest edges

Black (Details - Armour Ribbing, Cables, Gun casing):

1. Base with Abaddon Black
2. Highlight the edges with Eshin Grey
3. Highlight fine points and sharp edges with Dawnstone

Red (Details - Cloth, Wraps, Lenses):

1. Base with Mephiston Red
2. Shade the recesses with Agrax Earthshade
3. Roughly highlight the edges with Evil Sunz Scarlet
4. Add a fine highlight of Wild Rider Red
5. Add points of Fire Dragon Bright

Brown (Details - Leather Pouches):

1. Base with Rhinox Hide
2. Shade the recesses with Agrax Earthshade
3. Roughly highlight the edges with Skrag Brown
4. Add a fine highlight of Ushabti Bone