Stormfiend vs Green Shark Skin

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Green Shark Skin is a little darker, warmer than Stormfiend.

Can I use Green Shark Skin instead of Stormfiend?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Green Shark Skin is a little darker, warmer than Stormfiend.

Can I use Stormfiend instead of Green Shark Skin?

The same difference applies the other way round: Stormfiend is a little 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.