Fey Pink vs Smalt Hue

How close are they?

ΔE 35.6, loose: different enough that you should pick on purpose. Smalt Hue is a little darker, cooler than Fey Pink.

Can I use Smalt Hue instead of Fey Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Smalt Hue is a little darker, cooler than Fey Pink.

Can I use Fey Pink instead of Smalt Hue?

The same difference applies the other way round: Fey Pink is a little lighter, 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.