RLM 66 (1938) vs IJN Deep Dark Green

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. IJN Deep Dark Green is more saturated than RLM 66 (1938).

Can I use IJN Deep Dark Green instead of RLM 66 (1938)?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). IJN Deep Dark Green is more saturated than RLM 66 (1938).

Can I use RLM 66 (1938) instead of IJN Deep Dark Green?

The same difference applies the other way round: RLM 66 (1938) 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.