ΔE 12.8, fair: the same colour family, but not a straight swap. Flat Brown is a little lighter, more saturated, warmer than Nocturne Skin.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Flat Brown is a little lighter, more saturated, warmer than Nocturne Skin.
The same difference applies the other way round: Nocturne Skin is a little darker, more muted, cooler.
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.
See Nocturne Skin on the colour wheel See Flat Brown on the colour wheel
All matches: Nocturne Skin equivalents · Flat Brown equivalents