Holographic Paint vs Ultramarine Blue

How close are they?

ΔE 27.9, loose: different enough that you should pick on purpose. Ultramarine Blue is a little lighter, much more saturated than Holographic Paint.

Can I use Ultramarine Blue instead of Holographic Paint?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Ultramarine Blue is a little lighter, much more saturated than Holographic Paint.

Can I use Holographic Paint instead of Ultramarine Blue?

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