Aethermatic Blue vs Cool Blue

How close are they?

ΔE 27.7, loose: different enough that you should pick on purpose. Cool Blue is noticeably darker, warmer than Aethermatic Blue.

Can I use Cool Blue instead of Aethermatic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Cool Blue is noticeably darker, warmer than Aethermatic Blue.

Can I use Aethermatic Blue instead of Cool Blue?

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