Caledor Sky vs Prussian Blue

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Prussian Blue is noticeably darker, more muted, cooler than Caledor Sky.

Can I use Prussian Blue instead of Caledor Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Prussian Blue is noticeably darker, more muted, cooler than Caledor Sky.

Can I use Caledor Sky instead of Prussian Blue?

The same difference applies the other way round: Caledor Sky is noticeably lighter, more saturated, 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.