Corax White vs Underbelly Blue

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Underbelly Blue is a little darker, more saturated than Corax White.

Can I use Underbelly Blue instead of Corax White?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Underbelly Blue is a little darker, more saturated than Corax White.

Can I use Corax White instead of Underbelly Blue?

The same difference applies the other way round: Corax White is a little lighter, 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.