Runic Grey vs Leviathan Blue

How close are they?

ΔE 17.5, loose: different enough that you should pick on purpose. Leviathan Blue is noticeably darker, more saturated, warmer than Runic Grey.

Can I use Leviathan Blue instead of Runic Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Leviathan Blue is noticeably darker, more saturated, warmer than Runic Grey.

Can I use Runic Grey instead of Leviathan Blue?

The same difference applies the other way round: Runic Grey is noticeably lighter, 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.