Mordant Earth vs Burnt Umber

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Burnt Umber is a little darker than Mordant Earth.

Can I use Burnt Umber instead of Mordant Earth?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Burnt Umber is a little darker than Mordant Earth.

Can I use Mordant Earth instead of Burnt Umber?

The same difference applies the other way round: Mordant Earth is a little lighter.

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.