Hardened Carapace vs Burnt Sienna Umber

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Burnt Sienna Umber is a little lighter, more saturated than Hardened Carapace.

Can I use Burnt Sienna Umber instead of Hardened Carapace?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Burnt Sienna Umber is a little lighter, more saturated than Hardened Carapace.

Can I use Hardened Carapace instead of Burnt Sienna Umber?

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

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.