Outlining Black vs Night Blue

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Night Blue is a little lighter, more saturated than Outlining Black.

Can I use Night Blue instead of Outlining Black?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Night Blue is a little lighter, more saturated than Outlining Black.

Can I use Outlining Black instead of Night Blue?

The same difference applies the other way round: Outlining Black is a little darker, 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.