Burnt Umber vs Reddish Flesh

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Reddish Flesh is noticeably lighter, much more saturated, cooler than Burnt Umber.

Can I use Reddish Flesh instead of Burnt Umber?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Reddish Flesh is noticeably lighter, much more saturated, cooler than Burnt Umber.

Can I use Burnt Umber instead of Reddish Flesh?

The same difference applies the other way round: Burnt Umber is noticeably darker, much more muted, warmer.

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.