Blue Flux vs Twilight at Gettysburg

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Twilight at Gettysburg is a little lighter, warmer than Blue Flux.

Can I use Twilight at Gettysburg instead of Blue Flux?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Twilight at Gettysburg is a little lighter, warmer than Blue Flux.

Can I use Blue Flux instead of Twilight at Gettysburg?

The same difference applies the other way round: Blue Flux is a little 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.