Metallics are very hard to differentiate unless you put them side by side, so I did.
Left to right:
- 77.712 Steel
- 77.713 Jet Exhaust
- 77.720 Gunmetal
- 77.723 Exhaust Manifold
- 77.721 Burnt Iron
- Steel: Machined and unpainted steel, like it just rolled off the assembly line. This will show any wash or filter you put over it to weather - it is, as the tin suggests, a neutral steel color.
- Jet Exhaust: The above, but with a thin coat of Nuln Oil or similar black wash pre-applied. As the name would suggest this will give a more "used" feel to the metal. This is probably where you start and drybrush up to build layers for any oil-based engine or machinery.
- Gunmetal: Bright and polished steel. This is a great soft highlight color for any of the rest of the set without going all the way to a silver or aluminum. It's likely not a good basecoat color since you'd have to use an extremely bright highlight.
- Exhaust Manifold: A very faint coat of Agrax or similar brown wash over Steel that comes out just slightly less reflective and a little grimy. It's pretty hard to tell this apart from Steel other than the lower reflectivity, and even Jet Exhaust isn't much different - this is just brown-ish instead of black-ish. Of the five this is the "dirtiest" color due to the inclusion of a bit of brown, and is probably best for ruined or heavily weathered steel.
- Burnt Iron: A slightly less reflective Steel that still feels neutral, and a very close match for Leadbelcher. The brightness is about the same on a flat panel but not as pronounced on curved surfaces. You can probably get some subtle shading with this as a basecoat and a zenethil of Steel, but I'd be surprised if that's something that is perceptible on an infantry model unless you're a god tier expert in which case you're probably doing NMM anyway.