Olive Drab vs Pastel Green

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Pastel Green is a little lighter, more muted than Olive Drab.

Can I use Pastel Green instead of Olive Drab?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Pastel Green is a little lighter, more muted than Olive Drab.

Can I use Olive Drab instead of Pastel Green?

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