ΔE 12.7, fair: the same colour family, but not a straight swap. Ultramarine Blue is noticeably lighter, more muted than Eldar Robe.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Ultramarine Blue is noticeably lighter, more muted than Eldar Robe.
The same difference applies the other way round: Eldar Robe is noticeably darker, 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 Eldar Robe on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Eldar Robe equivalents · Ultramarine Blue equivalents