ΔE 13.6, fair: the same colour family, but not a straight swap. Mystic Blue is noticeably darker, more muted than Royal Robes.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Mystic Blue is noticeably darker, more muted than Royal Robes.
The same difference applies the other way round: Royal Robes is noticeably lighter, 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 Mystic Blue on the colour wheel
All matches: Royal Robes equivalents · Mystic Blue equivalents