Burnt Umber vs Black Grey

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Black Grey is a little lighter, more muted than Burnt Umber.

Can I use Black Grey instead of Burnt Umber?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Black Grey is a little lighter, more muted than Burnt Umber.

Can I use Burnt Umber instead of Black Grey?

The same difference applies the other way round: Burnt Umber is a little darker, 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.