Thrall Flesh vs Toxic Yellow

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Toxic Yellow is much more saturated, warmer than Thrall Flesh.

Can I use Toxic Yellow instead of Thrall Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Toxic Yellow is much more saturated, warmer than Thrall Flesh.

Can I use Thrall Flesh instead of Toxic Yellow?

The same difference applies the other way round: Thrall Flesh is much more muted, 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.