Decayed Metal vs Rusty Metal

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Rusty Metal is a little lighter, more muted than Decayed Metal.

Can I use Rusty Metal instead of Decayed Metal?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Rusty Metal is a little lighter, more muted than Decayed Metal.

Can I use Decayed Metal instead of Rusty Metal?

The same difference applies the other way round: Decayed Metal is a little darker, more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.