Spaceship Exterior vs Underbelly Blue

How close are they?

ΔE 7.7, close: similar, with a visible difference side by side. Underbelly Blue is more saturated than Spaceship Exterior.

Can I use Underbelly Blue instead of Spaceship Exterior?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Underbelly Blue is more saturated than Spaceship Exterior.

Can I use Spaceship Exterior instead of Underbelly Blue?

The same difference applies the other way round: Spaceship Exterior is 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.