Turquoise Ghost vs Space Grey

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Space Grey is a little lighter, more muted, cooler than Turquoise Ghost.

Can I use Space Grey instead of Turquoise Ghost?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Space Grey is a little lighter, more muted, cooler than Turquoise Ghost.

Can I use Turquoise Ghost instead of Space Grey?

The same difference applies the other way round: Turquoise Ghost is a little darker, more saturated, 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.