Solid Gold vs Radiant Yellow

How close are they?

ΔE 21.0, loose: different enough that you should pick on purpose. Radiant Yellow is noticeably darker, warmer than Solid Gold.

Can I use Radiant Yellow instead of Solid Gold?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Radiant Yellow is noticeably darker, warmer than Solid Gold.

Can I use Solid Gold instead of Radiant Yellow?

The same difference applies the other way round: Solid 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.