ΔE 9.9, close: similar, with a visible difference side by side. Black Grey is a little lighter, more muted than Umbral Umber.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Black Grey is a little lighter, more muted than Umbral Umber.
The same difference applies the other way round: Umbral 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.
See Umbral Umber on the colour wheel See Black Grey on the colour wheel
All matches: Umbral Umber equivalents · Black Grey equivalents