Stormfiend vs Indigo Blue

How close are they?

ΔE 23.7, loose: different enough that you should pick on purpose. Indigo Blue is noticeably darker, warmer than Stormfiend.

Can I use Indigo Blue instead of Stormfiend?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Indigo Blue is noticeably darker, warmer than Stormfiend.

Can I use Stormfiend instead of Indigo Blue?

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.