Aethermatic Blue vs Grey Mist

How close are they?

ΔE 34.8, loose: different enough that you should pick on purpose. Grey Mist is noticeably darker, much more muted than Aethermatic Blue.

Can I use Grey Mist instead of Aethermatic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). Grey Mist is noticeably darker, much more muted than Aethermatic Blue.

Can I use Aethermatic Blue instead of Grey Mist?

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