Gunmetal Blue vs Hydra Turquoise

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Hydra Turquoise is a little lighter, more saturated, warmer than Gunmetal Blue.

Can I use Hydra Turquoise instead of Gunmetal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Hydra Turquoise is a little lighter, more saturated, warmer than Gunmetal Blue.

Can I use Gunmetal Blue instead of Hydra Turquoise?

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