ΔE 15.8, loose: different enough that you should pick on purpose. Ghost Blue is a little lighter, warmer than Royal Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ghost Blue is a little lighter, warmer than Royal Blue.
The same difference applies the other way round: Royal Blue is a little darker, 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.
See Royal Blue on the colour wheel See Ghost Blue on the colour wheel
All matches: Royal Blue equivalents · Ghost Blue equivalents