Undead Flesh vs Dirty Grey

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Dirty Grey is noticeably darker, much more saturated than Undead Flesh.

Can I use Dirty Grey instead of Undead Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dirty Grey is noticeably darker, much more saturated than Undead Flesh.

Can I use Undead Flesh instead of Dirty Grey?

The same difference applies the other way round: Undead Flesh is noticeably lighter, much 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.