ΔE 6.8, close: similar, with a visible difference side by side. Ironclad Grey is a little lighter than Black Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ironclad Grey is a little lighter than Black Brown.
The same difference applies the other way round: Black Brown is a little darker.
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 Black Brown on the colour wheel See Ironclad Grey on the colour wheel
All matches: Black Brown equivalents · Ironclad Grey equivalents