Undead Flesh vs Interior Grey Green

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Interior Grey Green is a little darker, more muted, cooler than Undead Flesh.

Can I use Interior Grey Green instead of Undead Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Interior Grey Green is a little darker, more muted, cooler than Undead Flesh.

Can I use Undead Flesh instead of Interior Grey Green?

The same difference applies the other way round: Undead Flesh is a little lighter, more saturated, 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.