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. I used Dragon's Gold but whatever you want.
4. Base coat non metallic sections in Dark Sea Blue
5. Highlight with Turquoise
6. Highlight with Blue Green . You may prefer Light Blue Green for a less subtle highlight
7. Wash with Coelia Greenshade , base using Burnt Red
8. Final highlight using a mix of the original Blue Green highlight mixed in with 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.