Fairy Blood vs Legacy Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Legacy Blue is noticeably darker, more saturated than Fairy Blood.

Can I use Legacy Blue instead of Fairy Blood?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Legacy Blue is noticeably darker, more saturated than Fairy Blood.

Can I use Fairy Blood instead of Legacy Blue?

The same difference applies the other way round: Fairy Blood 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.