ΔE 19.1, loose: different enough that you should pick on purpose. Elysium Blue is noticeably lighter, warmer than Twilight Sky.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Elysium Blue is noticeably lighter, warmer than Twilight Sky.
The same difference applies the other way round: Twilight Sky is noticeably darker, cooler.
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 Twilight Sky on the colour wheel See Elysium Blue on the colour wheel
All matches: Twilight Sky equivalents · Elysium Blue equivalents