ΔE 10.0, fair: the same colour family, but not a straight swap. Red Grey is a little darker, cooler than Monster Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Red Grey is a little darker, cooler than Monster Brown.
The same difference applies the other way round: Monster Brown is a little lighter, 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 Red Grey on the colour wheel
All matches: Monster Brown equivalents · Red Grey equivalents