ΔE 12.1, fair: the same colour family, but not a straight swap. Onyx Skin is a little darker, more saturated, warmer than Grey Shadow.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Onyx Skin is a little darker, more saturated, warmer than Grey Shadow.
The same difference applies the other way round: Grey Shadow is a little 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.
See Grey Shadow on the colour wheel See Onyx Skin on the colour wheel
All matches: Grey Shadow equivalents · Onyx Skin equivalents