A combination of the video linked (for all the detail work), with the 'Armour of the Dark Gods' video by Duncan Rhoades for the metallic Tzeentch armour.

1. Prime Chaos Black

Main metallic armour (https://www.youtube.com/watch?v=UDvri5Aa1m8)

1. Cover all the armour pieces, not the armour trim however, with two thin coats of Stormhost Silver
2. Cover all the silver with Gryph-charger Grey , try to apply it evenly, as it will serve as a base texture for the other contrasts to stick to.
3. Now take Akhelian Green and thin it with a little bit of water, and do the same with Terradon Turquoise . Cover the armour one panel at a time by colouring it in with the Akhelian Green with some tiny splashes of Terradon Turquoise for the variety. Repeat this step if you feel the black is darkening the armour too much.
4. Pick out all of the armour trim with Retributor Armour Careful not to touch the blue armour.
5. Paint the metallics (exhausts, gunbarrels, tubes) with Leadbelcher

Further layering and details (https://www.youtube.com/watch?v=pIqHATHsll0)

1. Paint in the stripes of the headpiece and the legs or backpack with alternating Averland Sunset and Ahriman Blue
2. Add Mephiston Red to the various eyes / gems on the armour (usually on backpack, knees and chest piece)
3. Use White Scar for the eyes, as well as the cloth / cape. If the weapon has an area with runes you want to give a green warp glow later, paint that area as well.
4. Layer the yellow lines with a light bit of Yriel Yellow
5. Add a very thinned layer of Administratum Grey to add depth to the cloth / cape

Shading

1. Cover the gold armour with a light shade like Reikland Fleshshade (warmer tone) or Agrax Earthshade (more neutral)
2. And the silver with Nuln Oil
3. Use either Warp Lightning or Hexwraith Flame for the eyes, and the before mentioned runes
4. Use some Druchii Violet or Carroburg Crimson to cover the red eyes/gems.

Highlighting / final details (leave shades to dry properly before this step)

1. If applicable, recolour the weapon around the runes back to Abaddon Black , leaving the green in the recessed runes.
2. Bring back the main gold colour using another layer of Retributor Armour (avoid the recesses) and edge highlight it with Liberator Gold
3. Bring back the Leadbelcher
4. Highlight the sharpest edges of the blue armour and the gold trim, as well as the Leadbelcher with very thin Stormhost Silver
5. Reapply the Mephiston Red to the gems, following the shape and covering about 2/3rds from the bottom. Highlight this using a little streak of Evil Sunz Scarlet
6. Highlight the Ahriman Blue stripes using any lighter blue colour, such as Baharroth Blue
7. Add a tiny thin line of White Scar to the top of the red gemstones, as well as to the Yriel Yellow stripes. Also use it to bring back the white highlight to the cloth / cloak on the raised areas.