Stormfiend vs Dark Green Blue

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Dark Green Blue is noticeably darker, more muted, warmer than Stormfiend.

Can I use Dark Green Blue instead of Stormfiend?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Dark Green Blue is noticeably darker, more muted, warmer than Stormfiend.

Can I use Stormfiend instead of Dark Green Blue?

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