Ratskin Flesh vs Burnt Sienna

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Burnt Sienna is noticeably darker, more saturated than Ratskin Flesh.

Can I use Burnt Sienna instead of Ratskin Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Burnt Sienna is noticeably darker, more saturated than Ratskin Flesh.

Can I use Ratskin Flesh instead of Burnt Sienna?

The same difference applies the other way round: Ratskin 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.