ΔE 11.2, fair: the same colour family, but not a straight swap. Panzer Dark Grey is a little darker, more muted than Dark Reaper.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Panzer Dark Grey is a little darker, more muted than Dark Reaper.
The same difference applies the other way round: Dark Reaper is a little lighter, 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.
See Dark Reaper on the colour wheel See Panzer Dark Grey on the colour wheel
All matches: Dark Reaper equivalents · Panzer Dark Grey equivalents