Old Copper vs Radiant Yellow

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Radiant Yellow is a little lighter, more saturated, cooler than Old Copper.

Can I use Radiant Yellow instead of Old Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Radiant Yellow is a little lighter, more saturated, cooler than Old Copper.

Can I use Old Copper instead of Radiant Yellow?

The same difference applies the other way round: Old Copper is a little 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.