Dark Olive Drab vs Death Korps Drab

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Death Korps Drab is noticeably lighter, cooler than Dark Olive Drab.

Can I use Death Korps Drab instead of Dark Olive Drab?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Death Korps Drab is noticeably lighter, cooler than Dark Olive Drab.

Can I use Dark Olive Drab instead of Death Korps Drab?

The same difference applies the other way round: Dark Olive Drab is noticeably darker, warmer.

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.