Recipe from Tale of Painters Stahly for Heresy Era Alpha Legion. I'm making an entry here for ease of organization/reference. Original can be found here. https://taleofpainters.com/2022/07/tutorial-how-to-paint-alpha-legion-armour-heresy-era/

Priming

Prime model in silver. Midtone would probably be best, but darker steel or gunmetal might help with shadows/recesses.

Armor

1. Basecoat the armour with Shining Silver . If you don’t have an airbrush, you can also apply the regular Shining Silver version [or Stormhost Silver ].
2. Next, basecoat the armour with Azure Magic . This might take three thinned coats.
3. Tidy up the trims with Shining Silver [or Stormhost Silver ].
4. Create a mix of Dark Tone , :Bluestone, and Speedpaint Medium in a ratio of about 1:1:2, and apply it all over the armour and trims. The new formula Drakenhof Nightshade would also work very well for this.
5. If you find that certain areas didn’t receive a deep enough shading, paint Cloudburst Blue [or Leviadon Blue ] directly into the recesses where needed.
6. In case the wash left some unwanted pooling on the teal areas, mix about 8 parts of Azure Magic with a little bit of Night Scales and paint it over any uneven areas.
7. Carefully drybrush both the armour and trims with Shining Silver [or Stormhost Silver ] with a soft makeup brush, concentrating on the edges of the armour.
8. Thin Plasmatic Bolt 1:1 with Speedpaint Medium [or use pure Aethermatic Blue] and paint it all over the teal areas to knock down the Shining Silver highlights.

Vehicles

When painting vehicles, applying an all-over wash will result in a blotchy finish. Instead, add a little bit of Night Scales to your Azure Magic like in step 6 to match the armour colour of your Marines, and use it to basecoat the vehicle (it might be a good idea to create some more of this mix and save it in a spare bottle or pot to keep your army consistent). Then, use pure Cloudburst Blue Speedpaint [or Leviadon Blue] with a little bit of Speedpaint Medium [or Contrast Medium] and apply it directly into the recesses for shading. Then, complete the effect by drybrushing the armour with Shining Silver with a little bit of Azure Magic mixed in.