ΔE 22.4, loose: different enough that you should pick on purpose. Dark Prussian Blue is noticeably darker, more saturated, warmer than Greatcoat Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Dark Prussian Blue is noticeably darker, more saturated, warmer than Greatcoat Grey.
The same difference applies the other way round: Greatcoat Grey is noticeably lighter, 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 Dark Prussian Blue on the colour wheel
All matches: Greatcoat Grey equivalents · Dark Prussian Blue equivalents