Forge World Diemos has a pretty unusual colour scheme. Heres my take on it.

1. Priming in black (I like Vallejo mecha colour because it's a bit glossy and goes through the airbrush better)
2. Dry Brush or base coat in a steel. I used Sir Coates Silver but anything goes.
3. Dry brush trim in gold
4. Base coat non metallic sections in Dark Sea Blue
5. Highlight with Turquoise
6. Highlight with Blue Green
7. Wash with Coelia Greenshade , bae using Burnt Red
8. Final highlight using a mix of Blue Green and Ice Yellow but you may prefer using Nihilakh Oxide . Wash the base using a mix of red and orange oxide pigment powders, mixed into Vallejo airbrush thinner.