Dark Sky vs Deep Blue

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Deep Blue is a little darker, warmer than Dark Sky.

Can I use Deep Blue instead of Dark Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Deep Blue is a little darker, warmer than Dark Sky.

Can I use Dark Sky instead of Deep Blue?

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