Heartfire vs Dark Flesh

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Dark Flesh is noticeably darker, much more muted than Heartfire.

Can I use Dark Flesh instead of Heartfire?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Flesh is noticeably darker, much more muted than Heartfire.

Can I use Heartfire instead of Dark Flesh?

The same difference applies the other way round: Heartfire is noticeably lighter, much more saturated.

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.