ΔE 13.8, fair: the same colour family, but not a straight swap. Gray Green (Luftwaffe) is noticeably lighter, warmer than Incubi Darkness.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Gray Green (Luftwaffe) is noticeably lighter, warmer than Incubi Darkness.
The same difference applies the other way round: Incubi Darkness is noticeably darker, 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 Incubi Darkness on the colour wheel See Gray Green (Luftwaffe) on the colour wheel
All matches: Incubi Darkness equivalents · Gray Green (Luftwaffe) equivalents