Stormfang vs Cool Grey

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Cool Grey is noticeably lighter, more muted than Stormfang.

Can I use Cool Grey instead of Stormfang?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Cool Grey is noticeably lighter, more muted than Stormfang.

Can I use Stormfang instead of Cool Grey?

The same difference applies the other way round: Stormfang is noticeably darker, more saturated.

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.