Hardened Carapace vs Decay Black

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Decay Black is a little darker, more muted than Hardened Carapace.

Can I use Decay Black instead of Hardened Carapace?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Decay Black is a little darker, more muted than Hardened Carapace.

Can I use Hardened Carapace instead of Decay Black?

The same difference applies the other way round: Hardened Carapace is a little lighter, 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.