Hydra Turquoise vs Eldritch

How close are they?

ΔE 26.7, loose: different enough that you should pick on purpose. Eldritch is a little darker, more saturated, warmer than Hydra Turquoise.

Can I use Eldritch instead of Hydra Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Eldritch is a little darker, more saturated, warmer than Hydra Turquoise.

Can I use Hydra Turquoise instead of Eldritch?

The same difference applies the other way round: Hydra Turquoise is a little lighter, more muted, 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.