Pastel Indigo vs Aethermatic Blue

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Aethermatic Blue is a little darker, cooler than Pastel Indigo.

Can I use Aethermatic Blue instead of Pastel Indigo?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Aethermatic Blue is a little darker, cooler than Pastel Indigo.

Can I use Pastel Indigo instead of Aethermatic Blue?

The same difference applies the other way round: Pastel Indigo is a little lighter, 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.