ΔE 25.8, loose: different enough that you should pick on purpose. Greatcoat Grey is a little lighter, much more muted, cooler than Night Lords Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Greatcoat Grey is a little lighter, much more muted, cooler than Night Lords Blue.
The same difference applies the other way round: Night Lords Blue is a little darker, much more saturated, warmer.
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 Night Lords Blue on the colour wheel See Greatcoat Grey on the colour wheel
All matches: Night Lords Blue equivalents · Greatcoat Grey equivalents