Aethermatic Blue vs Grey Spell

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Grey Spell is a little darker, more muted, warmer than Aethermatic Blue.

Can I use Grey Spell instead of Aethermatic Blue?

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

Can I use Aethermatic Blue instead of Grey Spell?

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