Army Green vs Olive Green

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Olive Green is noticeably lighter, more saturated, cooler than Army Green.

Can I use Olive Green instead of Army Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Olive Green is noticeably lighter, more saturated, cooler than Army Green.

Can I use Army Green instead of Olive Green?

The same difference applies the other way round: Army Green is noticeably darker, more muted, 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.