ΔE 13.6, fair: the same colour family, but not a straight swap. Ultramarine Blue is a little darker, more muted 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.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Ultramarine Blue is a little darker, more muted than Pearl Blue. They are different kinds of paint.
The same difference applies the other way round: Pearl Blue 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.
See Pearl Blue on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Pearl Blue equivalents · Ultramarine Blue equivalents