ΔE 23.9, loose: different enough that you should pick on purpose. Medium Blue is a little darker, much more saturated, warmer than Greatcoat Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Medium Blue is a little darker, much more saturated, warmer than Greatcoat Grey.
The same difference applies the other way round: Greatcoat Grey is a little lighter, much more muted, cooler.
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 Greatcoat Grey on the colour wheel See Medium Blue on the colour wheel
All matches: Greatcoat Grey equivalents · Medium Blue equivalents