ΔE 6.7, close: similar, with a visible difference side by side. Ironclad Grey is a little lighter than NATO Black.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ironclad Grey is a little lighter than NATO Black.
The same difference applies the other way round: NATO Black 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 NATO Black on the colour wheel See Ironclad Grey on the colour wheel
All matches: NATO Black equivalents · Ironclad Grey equivalents