Phthalo Green vs Dark Drab

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Dark Drab is a little darker, more muted than Phthalo Green.

Can I use Dark Drab instead of Phthalo Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Drab is a little darker, more muted than Phthalo Green.

Can I use Phthalo Green instead of Dark Drab?

The same difference applies the other way round: Phthalo Green is a little lighter, 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.