Grey Blue vs Turquoise

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Turquoise is a little darker, much more saturated, cooler than Grey Blue.

Can I use Turquoise instead of Grey Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Turquoise is a little darker, much more saturated, cooler than Grey Blue.

Can I use Grey Blue instead of Turquoise?

The same difference applies the other way round: Grey Blue is a little lighter, much more muted, 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.