ΔE 10.4, fair: the same colour family, but not a straight swap. Steel Legion Drab is more saturated than RLM 66 (1941).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Steel Legion Drab is more saturated than RLM 66 (1941).
The same difference applies the other way round: RLM 66 (1941) is 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 (1941) on the colour wheel See Steel Legion Drab on the colour wheel
All matches: RLM 66 (1941) equivalents · Steel Legion Drab equivalents