Stormfiend vs Space Grey

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Space Grey is a little darker, more muted, warmer than Stormfiend.

Can I use Space Grey instead of Stormfiend?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Space Grey is a little darker, more muted, warmer than Stormfiend.

Can I use Stormfiend instead of Space Grey?

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