Dark Sky vs Marine Blue

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Marine Blue is noticeably lighter, more muted than Dark Sky.

Can I use Marine Blue instead of Dark Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Marine Blue is noticeably lighter, more muted than Dark Sky.

Can I use Dark Sky instead of Marine Blue?

The same difference applies the other way round: Dark Sky is noticeably darker, 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.