ΔE 12.7, fair: the same colour family, but not a straight swap. Flat Brown is a little lighter, warmer than Crimson Hell.
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, warmer than Crimson Hell.
The same difference applies the other way round: Crimson Hell is a little darker, 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 Crimson Hell on the colour wheel See Flat Brown on the colour wheel
All matches: Crimson Hell equivalents · Flat Brown equivalents