Dark Sky vs Macragge Blue

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Macragge Blue is a little lighter, much more saturated than Dark Sky.

Can I use Macragge Blue instead of Dark Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Macragge Blue is a little lighter, much more saturated than Dark Sky.

Can I use Dark Sky instead of Macragge Blue?

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

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.