Imperial Blue vs Dark Sky

How close are they?

ΔE 9.7, close: similar, with a visible difference side by side. Dark Sky is a little lighter, more muted than Imperial Blue.

Can I use Dark Sky instead of Imperial Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Dark Sky is a little lighter, more muted than Imperial Blue.

Can I use Imperial Blue instead of Dark Sky?

The same difference applies the other way round: Imperial Blue is a little 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.