ΔE 15.7, loose: different enough that you should pick on purpose. Twilight at Gettysburg is noticeably lighter, more saturated, warmer than Ionic Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Twilight at Gettysburg is noticeably lighter, more saturated, warmer than Ionic Blue.
The same difference applies the other way round: Ionic Blue is noticeably darker, more muted, 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 Ionic Blue on the colour wheel See Twilight at Gettysburg on the colour wheel
All matches: Ionic Blue equivalents · Twilight at Gettysburg equivalents