Stormfang vs Surf Aqua

How close are they?

ΔE 21.0, loose: different enough that you should pick on purpose. Surf Aqua is a little lighter, more saturated, warmer than Stormfang.

Can I use Surf Aqua instead of Stormfang?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Surf Aqua is a little lighter, more saturated, warmer than Stormfang.

Can I use Stormfang instead of Surf Aqua?

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.