Aethermatic Blue vs Space Grey

How close are they?

ΔE 35.6, loose: different enough that you should pick on purpose. Space Grey is noticeably darker, more muted, warmer than Aethermatic Blue.

Can I use Space Grey instead of Aethermatic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Space Grey is noticeably darker, more muted, warmer than Aethermatic Blue.

Can I use Aethermatic Blue instead of Space Grey?

The same difference applies the other way round: Aethermatic Blue is noticeably lighter, 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.