Runic Grey vs Dark Green Blue

How close are they?

ΔE 24.0, loose: different enough that you should pick on purpose. Dark Green Blue is noticeably darker, warmer than Runic Grey.

Can I use Dark Green Blue instead of Runic Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Dark Green Blue is noticeably darker, warmer than Runic Grey.

Can I use Runic Grey instead of Dark Green Blue?

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