Twilight Sky vs Caspian Blue

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Caspian Blue is a little lighter, much more muted, cooler than Twilight Sky.

Can I use Caspian Blue instead of Twilight Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Caspian Blue is a little lighter, much more muted, cooler than Twilight Sky.

Can I use Twilight Sky instead of Caspian Blue?

The same difference applies the other way round: Twilight Sky is a little darker, much more saturated, warmer.

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.