Stormfang vs Hiril Blue

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Hiril Blue is a little lighter, more saturated, warmer than Stormfang.

Can I use Hiril Blue instead of Stormfang?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Hiril Blue is a little lighter, more saturated, warmer than Stormfang.

Can I use Stormfang instead of Hiril Blue?

The same difference applies the other way round: Stormfang is a little darker, more muted, 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.