ΔE 9.7, close: similar, with a visible difference side by side. Transparent Blue is a little lighter than Ultramarines Blue. They are different kinds of paint: Citadel Ultramarines Blue is a one-coat contrast-style paint and Pro Acryl Transparent Blue is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Transparent Blue is a little lighter than Ultramarines Blue. They are different kinds of paint.
The same difference applies the other way round: Ultramarines Blue is a little darker.
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 Ultramarines Blue on the colour wheel See Transparent Blue on the colour wheel
All matches: Ultramarines Blue equivalents · Transparent Blue equivalents