ΔE 12.0, fair: the same colour family, but not a straight swap. Thousand Sons Blue is a little lighter, more saturated, cooler than Ultramarine Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Thousand Sons Blue is a little lighter, more saturated, cooler than Ultramarine Blue.
The same difference applies the other way round: Ultramarine Blue is a little darker, more muted, warmer.
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 Ultramarine Blue on the colour wheel See Thousand Sons Blue on the colour wheel
All matches: Ultramarine Blue equivalents · Thousand Sons Blue equivalents