Aquamarine vs Underbelly Blue

How close are they?

ΔE 25.5, loose: different enough that you should pick on purpose. Underbelly Blue is noticeably lighter, much more muted than Aquamarine.

Can I use Underbelly Blue instead of Aquamarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Underbelly Blue is noticeably lighter, much more muted than Aquamarine.

Can I use Aquamarine instead of Underbelly Blue?

The same difference applies the other way round: Aquamarine is noticeably darker, 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.