ΔE 15.9, loose: different enough that you should pick on purpose. Cokum Copper is noticeably lighter, more saturated, cooler than Golfag Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Cokum Copper is noticeably lighter, more saturated, cooler than Golfag Brown.
The same difference applies the other way round: Golfag Brown is noticeably darker, 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 Golfag Brown on the colour wheel See Cokum Copper on the colour wheel
All matches: Golfag Brown equivalents · Cokum Copper equivalents