Dark Rust vs Dark Umber

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Dark Umber is a little lighter, cooler than Dark Rust.

Can I use Dark Umber instead of Dark Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Dark Umber is a little lighter, cooler than Dark Rust.

Can I use Dark Rust instead of Dark Umber?

The same difference applies the other way round: Dark Rust is a little darker, warmer.

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.