Iceborn vs Underbelly Blue

How close are they?

ΔE 20.3, loose: different enough that you should pick on purpose. Underbelly Blue is a little darker, more muted than Iceborn.

Can I use Underbelly Blue instead of Iceborn?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Underbelly Blue is a little darker, more muted than Iceborn.

Can I use Iceborn instead of Underbelly Blue?

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

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.