Olive Drab vs Bastion Grey

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Bastion Grey is a little darker, much more muted than Olive Drab.

Can I use Bastion Grey instead of Olive Drab?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Bastion Grey is a little darker, much more muted than Olive Drab.

Can I use Olive Drab instead of Bastion Grey?

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

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.