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

1. Prime with Chaos Black
2. Prime over that with Leadbelcher

Silver (Armour Trim)

1. Base with Leadbelcher
2. Shade the recesses with Nuln Oil
3. Roughly highlight the edges with Ironbreaker
4. Highlight fine points and sharp edges with Stormhost Silver

For grenades, replace Nuln Oil with Athonian Camoshade

Black (Armour Panels)

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

Grey (Armour Ribbing)

1. Base with Mechanicus Standard Grey
2. Shade the recesses with Nuln Oil
3. Highlight the edges with Dawnstone

Red (Cloth)

1. Base with Mephiston Red
2. Shade the recesses with Agrax Earthshade
3. Highlight the edges with Evil Sunz Scarlet
4. Finer highlight with Wild Rider Red

Tan (Details)

1. Base with Ushabti Bone
2. Shade the recesses with Agrax Earthshade
3. Highlight the edges with Screaming Skull

Gold (Details)

1. Base with Retributor Armour
2. Shade the recesses with Agrax Earthshade
3. Highlight the edges with Stormhost Silver

Brown (Leather)

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

Blue (Plasma)

1. Base with Sotek Green
2. Shade the recesses with Drakenhof Nightshade
3. Highlight the edges with Temple Guard Blue
4. Finer highlight with Baharroth Blue
5. Highlight fine points with Baharroth Blue mixed 1:1 with White Scar