ΔE 9.4, close: similar, with a visible difference side by side. Great Hall Grey is a little lighter, more saturated than Medium Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Great Hall Grey is a little lighter, more saturated than Medium Grey.
The same difference applies the other way round: Medium Grey is a little darker, more muted.
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 Medium Grey on the colour wheel See Great Hall Grey on the colour wheel
All matches: Medium Grey equivalents · Great Hall Grey equivalents