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. Highlight the edges with Stormhost Silver

For grenades, replace Nuln Oil with Athonian Camoshade

For a rust effect, layer Skrag Brown thinned with water

Purple (Armour Panels)

1. Base with Naggaroth Night
2. Shade the recesses with Nuln Oil
3. Highlight the edges with Xereus Purple

Black (Armour Ribbing)

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

Light Grey (Cloth)

1. Base with Celestra Grey
2. Shade the recesses with Drakenhof Nightshade
3. Highlight the edges with Ulthuan Grey

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

Red (Lenses)

1. Base with Mephiston Red
2. Shade the recesses with Agrax Earthshade
3. Layer the middle with Evil Sunz Scarlet
4. Highlight with Wild Rider Red
5. Point highlight with Fire Dragon Bright

Yellow (Details)

1. Base with Averland Sunset
2. Shade with Agrax Earthshade
3. Highlight the edges with Yriel Yellow

Green (Details)

1. Base with Warpstone Glow
2. Shade the recesses with Nuln Oil
3. Highlight the edges with Moot Green