Zombie Flesh vs Gray Green

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Gray Green is a little darker, cooler than Zombie Flesh.

Can I use Gray Green instead of Zombie Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Gray Green is a little darker, cooler than Zombie Flesh.

Can I use Zombie Flesh instead of Gray Green?

The same difference applies the other way round: Zombie Flesh is a little lighter, warmer.

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.