ΔE 10.3, fair: the same colour family, but not a straight swap. Walnut is a little darker, warmer than Monster Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Walnut is a little darker, warmer than Monster Brown.
The same difference applies the other way round: Monster 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 Monster Brown on the colour wheel See Walnut on the colour wheel
All matches: Monster Brown equivalents · Walnut equivalents