ΔE 15.9, loose: different enough that you should pick on purpose. Flat Brown is noticeably darker, more saturated, cooler than March of the Fire Ants.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Flat Brown is noticeably darker, more saturated, cooler than March of the Fire Ants.
The same difference applies the other way round: March of the Fire Ants is noticeably lighter, more muted, 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.
See March of the Fire Ants on the colour wheel See Flat Brown on the colour wheel
All matches: March of the Fire Ants equivalents · Flat Brown equivalents