ΔE 11.5, fair: the same colour family, but not a straight swap. Copper Brown is a little darker, more muted, cooler than Dark Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Copper Brown is a little darker, more muted, cooler than Dark Brown.
The same difference applies the other way round: Dark Brown is a little lighter, more saturated, 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 Dark Brown on the colour wheel See Copper Brown on the colour wheel
All matches: Dark Brown equivalents · Copper Brown equivalents