Mystic Gold vs Radiant Yellow

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Radiant Yellow is noticeably darker, more saturated, warmer than Mystic Gold.

Can I use Radiant Yellow instead of Mystic Gold?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Radiant Yellow is noticeably darker, more saturated, warmer than Mystic Gold.

Can I use Mystic Gold instead of Radiant Yellow?

The same difference applies the other way round: Mystic Gold is noticeably lighter, more muted, 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.