Arcane Blue vs Aquamarine

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Aquamarine is a little darker, much more saturated, cooler than Arcane Blue.

Can I use Aquamarine instead of Arcane Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Aquamarine is a little darker, much more saturated, cooler than Arcane Blue.

Can I use Arcane Blue instead of Aquamarine?

The same difference applies the other way round: Arcane Blue is a little lighter, much more muted, 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.