ΔE 57.5, loose: different enough that you should pick on purpose. Radiant Yellow is noticeably lighter, cooler than Copper.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 57). Radiant Yellow is noticeably lighter, cooler than Copper.
The same difference applies the other way round: Copper is noticeably darker, 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 Copper on the colour wheel See Radiant Yellow on the colour wheel
All matches: Copper equivalents · Radiant Yellow equivalents