Olive Drab vs Viridian Green

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Viridian Green is a little lighter, much more muted, cooler than Olive Drab.

Can I use Viridian Green instead of Olive Drab?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Viridian Green is a little lighter, much more muted, cooler than Olive Drab.

Can I use Olive Drab instead of Viridian Green?

The same difference applies the other way round: Olive Drab is a little darker, much more saturated, 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.