Stormfiend vs Fairy Blood

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Fairy Blood is noticeably darker, warmer than Stormfiend.

Can I use Fairy Blood instead of Stormfiend?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Fairy Blood is noticeably darker, warmer than Stormfiend.

Can I use Stormfiend instead of Fairy Blood?

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