Dark Sky vs Nocturne Skin

How close are they?

ΔE 20.9, loose: different enough that you should pick on purpose. Nocturne Skin is a little lighter, more muted, warmer than Dark Sky.

Can I use Nocturne Skin instead of Dark Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Nocturne Skin is a little lighter, more muted, warmer than Dark Sky.

Can I use Dark Sky instead of Nocturne Skin?

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