Knight-Questor Flesh vs Bestial Brown

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Bestial Brown is noticeably darker, warmer than Knight-Questor Flesh.

Can I use Bestial Brown instead of Knight-Questor Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Bestial Brown is noticeably darker, warmer than Knight-Questor Flesh.

Can I use Knight-Questor Flesh instead of Bestial Brown?

The same difference applies the other way round: Knight-Questor Flesh is noticeably lighter, 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.