Cosmic Dust vs Fairy Flesh

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Fairy Flesh is a little darker, warmer than Cosmic Dust.

Can I use Fairy Flesh instead of Cosmic Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Fairy Flesh is a little darker, warmer than Cosmic Dust.

Can I use Cosmic Dust instead of Fairy Flesh?

The same difference applies the other way round: Cosmic Dust is a little 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.