ΔE 19.8, loose: different enough that you should pick on purpose. Teclis Blue is noticeably darker than Twilight at Gettysburg.
Can I use Teclis Blue instead of Twilight at Gettysburg?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Teclis Blue is noticeably darker than Twilight at Gettysburg.
Can I use Twilight at Gettysburg instead of Teclis Blue?
The same difference applies the other way round: Twilight at Gettysburg is noticeably lighter.
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.