Stormfiend vs Caribbean Turquoise

How close are they?

ΔE 16.9, loose: different enough that you should pick on purpose. Caribbean Turquoise is noticeably darker, more muted than Stormfiend.

Can I use Caribbean Turquoise instead of Stormfiend?

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

Can I use Stormfiend instead of Caribbean Turquoise?

The same difference applies the other way round: Stormfiend is noticeably lighter, 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.