ΔE 14.5, fair: the same colour family, but not a straight swap. Transparent Blue is a little darker, more saturated, warmer than Calgar Blue. They are different kinds of paint: Citadel Calgar Blue is an opaque base/layer 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.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Transparent Blue is a little darker, more saturated, warmer than Calgar Blue. They are different kinds of paint.
The same difference applies the other way round: Calgar Blue is a little lighter, more muted, cooler.
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 Calgar Blue on the colour wheel See Transparent Blue on the colour wheel
All matches: Calgar Blue equivalents · Transparent Blue equivalents