Ferocious Green vs Thrall Flesh

How close are they?

ΔE 28.5, loose: different enough that you should pick on purpose. Thrall Flesh is noticeably lighter, much more muted, warmer than Ferocious Green.

Can I use Thrall Flesh instead of Ferocious Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Thrall Flesh is noticeably lighter, much more muted, warmer than Ferocious Green.

Can I use Ferocious Green instead of Thrall Flesh?

The same difference applies the other way round: Ferocious Green is noticeably darker, much more saturated, cooler.

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.