ΔE 20.5, loose: different enough that you should pick on purpose. Brutal Brown is noticeably darker, much more saturated, cooler than Monster Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Brutal Brown is noticeably darker, much more saturated, cooler than Monster Brown.
The same difference applies the other way round: Monster Brown is noticeably 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 Monster Brown on the colour wheel See Brutal Brown on the colour wheel
All matches: Monster Brown equivalents · Brutal Brown equivalents