ΔE 9.2, close: similar, with a visible difference side by side. Nocturna Shadow is a little darker than Grey Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Nocturna Shadow is a little darker than Grey Brown.
The same difference applies the other way round: Grey Brown 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.
See Grey Brown on the colour wheel See Nocturna Shadow on the colour wheel
All matches: Grey Brown equivalents · Nocturna Shadow equivalents