Stormfang vs Bluebird Grey

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Bluebird Grey is noticeably darker, warmer than Stormfang.

Can I use Bluebird Grey instead of Stormfang?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Bluebird Grey is noticeably darker, warmer than Stormfang.

Can I use Stormfang instead of Bluebird Grey?

The same difference applies the other way round: Stormfang 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.