Aegis Aqua vs Grey Blue

How close are they?

ΔE 20.9, loose: different enough that you should pick on purpose. Grey Blue is a little darker, much more muted, warmer than Aegis Aqua.

Can I use Grey Blue instead of Aegis Aqua?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Grey Blue is a little darker, much more muted, warmer than Aegis Aqua.

Can I use Aegis Aqua instead of Grey Blue?

The same difference applies the other way round: Aegis Aqua is a little 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.