Gravelord Grey vs Leviathan Blue

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Leviathan Blue is a little darker, much more saturated than Gravelord Grey.

Can I use Leviathan Blue instead of Gravelord Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Leviathan Blue is a little darker, much more saturated than Gravelord Grey.

Can I use Gravelord Grey instead of Leviathan Blue?

The same difference applies the other way round: Gravelord Grey is a little lighter, much more muted.

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.