ΔE 18.9, loose: different enough that you should pick on purpose. Royal Blue is a little darker, much more muted, cooler than Kingdom Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Royal Blue is a little darker, much more muted, cooler than Kingdom Blue.
The same difference applies the other way round: Kingdom Blue is a little lighter, much 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.
See Kingdom Blue on the colour wheel See Royal Blue on the colour wheel
All matches: Kingdom Blue equivalents · Royal Blue equivalents