Dark Feldgrau vs Green RLM73

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Green RLM73 is a little lighter, more muted than Dark Feldgrau.

Can I use Green RLM73 instead of Dark Feldgrau?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Green RLM73 is a little lighter, more muted than Dark Feldgrau.

Can I use Dark Feldgrau instead of Green RLM73?

The same difference applies the other way round: Dark Feldgrau is a little darker, 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.