Indigo Sky vs Imperial Blue

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Imperial Blue is a little darker, cooler than Indigo Sky.

Can I use Imperial Blue instead of Indigo Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Imperial Blue is a little darker, cooler than Indigo Sky.

Can I use Indigo Sky instead of Imperial Blue?

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

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.