Thrall Flesh vs UK BSC

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. UK BSC is noticeably darker, more muted, warmer than Thrall Flesh.

Can I use UK BSC instead of Thrall Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). UK BSC is noticeably darker, more muted, warmer than Thrall Flesh.

Can I use Thrall Flesh instead of UK BSC?

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