Elysian Green vs Greenskin Flesh

How close are they?

ΔE 23.4, loose: different enough that you should pick on purpose. Greenskin Flesh is noticeably darker, cooler than Elysian Green.

Can I use Greenskin Flesh instead of Elysian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Greenskin Flesh is noticeably darker, cooler than Elysian Green.

Can I use Elysian Green instead of Greenskin Flesh?

The same difference applies the other way round: Elysian Green is noticeably 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.