ΔE 9.4, close: similar, with a visible difference side by side. Dark Reaper is a little lighter, more saturated than RLM 66 (1938).
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Dark Reaper is a little lighter, more saturated than RLM 66 (1938).
The same difference applies the other way round: RLM 66 (1938) is a little darker, more muted.
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 RLM 66 (1938) on the colour wheel See Dark Reaper on the colour wheel
All matches: RLM 66 (1938) equivalents · Dark Reaper equivalents