Green Pearl vs Ultramarine Blue

How close are they?

ΔE 22.7, loose: different enough that you should pick on purpose. Ultramarine Blue is a little lighter, more saturated, cooler than Green Pearl.

Can I use Ultramarine Blue instead of Green Pearl?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Ultramarine Blue is a little lighter, more saturated, cooler than Green Pearl.

Can I use Green Pearl instead of Ultramarine Blue?

The same difference applies the other way round: Green Pearl is a little darker, more muted, warmer.

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.