ΔE 26.7, loose: different enough that you should pick on purpose. Indigo Blue is noticeably darker, much more muted than Royal Robes.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Indigo Blue is noticeably darker, much more muted than Royal Robes.
The same difference applies the other way round: Royal Robes is noticeably lighter, much more saturated.
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 Robes on the colour wheel See Indigo Blue on the colour wheel
All matches: Royal Robes equivalents · Indigo Blue equivalents