Indigo Sky vs Flat Blue

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Flat Blue is a little darker, more muted, cooler than Indigo Sky.

Can I use Flat Blue instead of Indigo Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Blue is a little darker, more muted, cooler than Indigo Sky.

Can I use Indigo Sky instead of Flat Blue?

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