Twilight at Gettysburg vs Ultramarine Blue

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably darker, more muted than Twilight at Gettysburg.

Can I use Ultramarine Blue instead of Twilight at Gettysburg?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Ultramarine Blue is noticeably darker, more muted than Twilight at Gettysburg.

Can I use Twilight at Gettysburg instead of Ultramarine Blue?

The same difference applies the other way round: Twilight at Gettysburg is noticeably lighter, more saturated.

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.