ΔE 39.5, loose: different enough that you should pick on purpose. Copper is noticeably darker, warmer than Thallax Gold.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 39). Copper is noticeably darker, warmer than Thallax Gold.
The same difference applies the other way round: Thallax Gold is noticeably 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 Thallax Gold on the colour wheel See Copper on the colour wheel
All matches: Thallax Gold equivalents · Copper equivalents