Barbarian Flesh vs Khardic Flesh

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Khardic Flesh is noticeably darker, more saturated than Barbarian Flesh.

Can I use Khardic Flesh instead of Barbarian Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Khardic Flesh is noticeably darker, more saturated than Barbarian Flesh.

Can I use Barbarian Flesh instead of Khardic Flesh?

The same difference applies the other way round: Barbarian Flesh is noticeably lighter, more muted.

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.