Stormy Sea vs Black Green

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Black Green is a little darker, more muted than Stormy Sea.

Can I use Black Green instead of Stormy Sea?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Black Green is a little darker, more muted than Stormy Sea.

Can I use Stormy Sea instead of Black Green?

The same difference applies the other way round: Stormy Sea is a little lighter, 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.