ΔE 6.8, close: similar, with a visible difference side by side. Dark Drab is a little darker, more saturated than Lead Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Dark Drab is a little darker, more saturated than Lead Grey.
The same difference applies the other way round: Lead Grey is a little lighter, more muted.
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.
See Lead Grey on the colour wheel See Dark Drab on the colour wheel
All matches: Lead Grey equivalents · Dark Drab equivalents