Dark Flesh vs Flat Brown

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Flat Brown is more muted, cooler than Dark Flesh.

Can I use Flat Brown instead of Dark Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Brown is more muted, cooler than Dark Flesh.

Can I use Dark Flesh instead of Flat Brown?

The same difference applies the other way round: Dark Flesh is more saturated, 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.