Raging Sea vs Leviathan Blue

How close are they?

ΔE 30.0, loose: different enough that you should pick on purpose. Leviathan Blue is noticeably darker, more muted, cooler than Raging Sea.

Can I use Leviathan Blue instead of Raging Sea?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Leviathan Blue is noticeably darker, more muted, cooler than Raging Sea.

Can I use Raging Sea instead of Leviathan Blue?

The same difference applies the other way round: Raging Sea is noticeably lighter, more saturated, 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.