ΔE 19.9, loose: different enough that you should pick on purpose. Coal Black is noticeably darker, warmer than Runic Grey. They are different kinds of paint: Army Painter Runic Grey is a one-coat contrast-style paint and Formula P3 Coal Black is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Coal Black is noticeably darker, warmer than Runic Grey. They are different kinds of paint.
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.
See Runic Grey on the colour wheel See Coal Black on the colour wheel
All matches: Runic Grey equivalents · Coal Black equivalents