ΔE 20.0, loose: different enough that you should pick on purpose. Stormfiend is noticeably lighter, more saturated, cooler than Turnbull Turquoise.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Stormfiend is noticeably lighter, more saturated, cooler than Turnbull Turquoise.
The same difference applies the other way round: Turnbull Turquoise is noticeably darker, more muted, warmer.
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.
See Turnbull Turquoise on the colour wheel See Stormfiend on the colour wheel
All matches: Turnbull Turquoise equivalents · Stormfiend equivalents