ΔE 13.3, fair: the same colour family, but not a straight swap. Dark Ghost Gray is much more muted than Skink Blue.
Can I use Dark Ghost Gray instead of Skink Blue?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Dark Ghost Gray is much more muted than Skink Blue.
Can I use Skink Blue instead of Dark Ghost Gray?
The same difference applies the other way round: Skink Blue is much more saturated.
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.