Neptunus Blue vs Hydra Turquoise

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Hydra Turquoise is a little lighter, more muted, warmer than Neptunus Blue.

Can I use Hydra Turquoise instead of Neptunus Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Hydra Turquoise is a little lighter, more muted, warmer than Neptunus Blue.

Can I use Neptunus Blue instead of Hydra Turquoise?

The same difference applies the other way round: Neptunus Blue is a little darker, 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.