ΔE 21.6, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably lighter, much more muted, cooler than Pearl Blue. They are different kinds of paint: Tamiya Pearl Blue is a pearl paint and Vallejo Ultramarine Blue is a metallic, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Ultramarine Blue is noticeably lighter, much more muted, cooler than Pearl Blue. They are different kinds of paint.
The same difference applies the other way round: Pearl Blue is noticeably darker, much more saturated, 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.
See Pearl Blue on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Pearl Blue equivalents · Ultramarine Blue equivalents