ΔE 24.6, loose: different enough that you should pick on purpose. Greatcoat Grey is noticeably darker, much more muted, warmer than Temple Guard Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Greatcoat Grey is noticeably darker, much more muted, warmer than Temple Guard Blue.
The same difference applies the other way round: Temple Guard Blue is noticeably lighter, much more saturated, 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 Temple Guard Blue on the colour wheel See Greatcoat Grey on the colour wheel
All matches: Temple Guard Blue equivalents · Greatcoat Grey equivalents