Dark Umber vs Flat Earth

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Flat Earth is a little lighter, more saturated, warmer than Dark Umber.

Can I use Flat Earth instead of Dark Umber?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Earth is a little lighter, more saturated, warmer than Dark Umber.

Can I use Dark Umber instead of Flat Earth?

The same difference applies the other way round: Dark Umber is a little darker, more muted, cooler.

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.