ΔE 21.6, loose: different enough that you should pick on purpose. Neptune's Fury is noticeably lighter, much more saturated, warmer than Royal Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Neptune's Fury is noticeably lighter, much more saturated, warmer than Royal Blue.
The same difference applies the other way round: Royal Blue is noticeably darker, much more muted, 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 Neptune's Fury on the colour wheel
All matches: Royal Blue equivalents · Neptune's Fury equivalents