Deep Azure vs Greatcoat Grey

How close are they?

ΔE 21.7, loose: different enough that you should pick on purpose. Greatcoat Grey is a little darker, much more muted than Deep Azure.

Can I use Greatcoat Grey instead of Deep Azure?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Greatcoat Grey is a little darker, much more muted than Deep Azure.

Can I use Deep Azure instead of Greatcoat Grey?

The same difference applies the other way round: Deep Azure is a little lighter, much more saturated.

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.