Dark Feldgrau vs Olive Green RLM80

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Olive Green RLM80 is a little lighter, warmer than Dark Feldgrau.

Can I use Olive Green RLM80 instead of Dark Feldgrau?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Olive Green RLM80 is a little lighter, warmer than Dark Feldgrau.

Can I use Dark Feldgrau instead of Olive Green RLM80?

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

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.