ΔE 14.7, fair: the same colour family, but not a straight swap. Elysium Blue is noticeably lighter than Thousand Sons Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Elysium Blue is noticeably lighter than Thousand Sons Blue.
The same difference applies the other way round: Thousand Sons Blue is noticeably darker.
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 Thousand Sons Blue on the colour wheel See Elysium Blue on the colour wheel
All matches: Thousand Sons Blue equivalents · Elysium Blue equivalents