ΔE 11.4, fair: the same colour family, but not a straight swap. Runic Grey is noticeably lighter than Shadow Grey. They are different kinds of paint: AMMO by Mig Shadow Grey is an opaque base/layer paint and Army Painter Runic Grey is a one-coat contrast-style paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Runic Grey is noticeably lighter than Shadow Grey. They are different kinds of paint.
The same difference applies the other way round: Shadow Grey is noticeably darker.
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 Shadow Grey on the colour wheel See Runic Grey on the colour wheel
All matches: Shadow Grey equivalents · Runic Grey equivalents