ΔE 10.4, fair: the same colour family, but not a straight swap. Flat Flesh is a little darker, warmer than Japanese Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Flat Flesh is a little darker, warmer than Japanese Brown.
The same difference applies the other way round: Japanese Brown is a little lighter, 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 Japanese Brown on the colour wheel See Flat Flesh on the colour wheel
All matches: Japanese Brown equivalents · Flat Flesh equivalents