WWI French Green 1 vs Eshin Grey

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Eshin Grey is a little lighter, much more muted than WWI French Green 1.

Can I use Eshin Grey instead of WWI French Green 1?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Eshin Grey is a little lighter, much more muted than WWI French Green 1.

Can I use WWI French Green 1 instead of Eshin Grey?

The same difference applies the other way round: WWI French Green 1 is a little darker, 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.