Prussian Blue Hue vs French Blue

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. French Blue is noticeably lighter, cooler than Prussian Blue Hue.

Can I use French Blue instead of Prussian Blue Hue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). French Blue is noticeably lighter, cooler than Prussian Blue Hue.

Can I use Prussian Blue Hue instead of French Blue?

The same difference applies the other way round: Prussian Blue Hue is noticeably darker, 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.