ΔE 24.2, loose: different enough that you should pick on purpose. Flat Red is a little darker, much more saturated, cooler than Brutal Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Flat Red is a little darker, much more saturated, cooler than Brutal Brown.
The same difference applies the other way round: Brutal Brown is a little lighter, much 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 Brutal Brown on the colour wheel See Flat Red on the colour wheel
All matches: Brutal Brown equivalents · Flat Red equivalents