Neptune Glow vs Underbelly Blue

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Underbelly Blue is a little lighter, much more muted than Neptune Glow.

Can I use Underbelly Blue instead of Neptune Glow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Underbelly Blue is a little lighter, much more muted than Neptune Glow.

Can I use Neptune Glow instead of Underbelly Blue?

The same difference applies the other way round: Neptune Glow is a little darker, much 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.