Dark Reaper vs Green FS34079

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Green FS34079 is a little lighter, warmer than Dark Reaper.

Can I use Green FS34079 instead of Dark Reaper?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Green FS34079 is a little lighter, warmer than Dark Reaper.

Can I use Dark Reaper instead of Green FS34079?

The same difference applies the other way round: Dark Reaper 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.