Goblin Flesh vs Sick Green

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Sick Green is noticeably darker, cooler than Goblin Flesh.

Can I use Sick Green instead of Goblin Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Sick Green is noticeably darker, cooler than Goblin Flesh.

Can I use Goblin Flesh instead of Sick Green?

The same difference applies the other way round: Goblin Flesh 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.