Fairy Dust vs Radiant Platinum

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Radiant Platinum is noticeably darker, more saturated than Fairy Dust.

Can I use Radiant Platinum instead of Fairy Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Radiant Platinum is noticeably darker, more saturated than Fairy Dust.

Can I use Fairy Dust instead of Radiant Platinum?

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

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.