Stormfang vs Dark Turquoise

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Dark Turquoise is noticeably darker, more saturated than Stormfang.

Can I use Dark Turquoise instead of Stormfang?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Dark Turquoise is noticeably darker, more saturated than Stormfang.

Can I use Stormfang instead of Dark Turquoise?

The same difference applies the other way round: Stormfang is noticeably lighter, more muted.

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.