Royal Blue vs Leviathan Blue

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Leviathan Blue is noticeably lighter, more muted, cooler than Royal Blue.

Can I use Leviathan Blue instead of Royal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Leviathan Blue is noticeably lighter, more muted, cooler than Royal Blue.

Can I use Royal Blue instead of Leviathan Blue?

The same difference applies the other way round: Royal Blue is noticeably darker, 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.