Prussian Blue Hue vs Royal Purple

How close are they?

ΔE 18.1, loose: different enough that you should pick on purpose. Royal Purple is a little lighter, more muted, warmer than Prussian Blue Hue.

Can I use Royal Purple instead of Prussian Blue Hue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Royal Purple is a little lighter, more muted, warmer than Prussian Blue Hue.

Can I use Prussian Blue Hue instead of Royal Purple?

The same difference applies the other way round: Prussian Blue Hue is a little darker, more saturated, 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.