ΔE 10.5, fair: the same colour family, but not a straight swap. Transparent Blue is more saturated, warmer than Royal Blue. They are different kinds of paint: Army Painter Royal 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 11). Transparent Blue is more saturated, warmer than Royal Blue. They are different kinds of paint.
The same difference applies the other way round: Royal Blue is 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 Royal Blue on the colour wheel See Transparent Blue on the colour wheel
All matches: Royal Blue equivalents · Transparent Blue equivalents