Runic Cobalt vs Stormfang

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Stormfang is a little darker, cooler than Runic Cobalt.

Can I use Stormfang instead of Runic Cobalt?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Stormfang is a little darker, cooler than Runic Cobalt.

Can I use Runic Cobalt instead of Stormfang?

The same difference applies the other way round: Runic Cobalt is a little lighter, warmer.

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.