Abyssal Blue vs Greatcoat Grey

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Greatcoat Grey is much more muted, warmer than Abyssal Blue.

Can I use Greatcoat Grey instead of Abyssal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Greatcoat Grey is much more muted, warmer than Abyssal Blue.

Can I use Abyssal Blue instead of Greatcoat Grey?

The same difference applies the other way round: Abyssal Blue is 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.