ΔE 12.1, fair: the same colour family, but not a straight swap. Blue is a little lighter, more saturated than Royal Blue. They are different kinds of paint: Army Painter Royal Blue is an opaque base/layer paint and Pro Acryl Blue is a one-coat contrast-style paint, so they won’t behave the same on the model even where the colour is close.
✓ Herrick Games paint comparison chart lists these two as equivalents.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Blue is a little lighter, more saturated than Royal Blue. They are different kinds of paint.
The same difference applies the other way round: Royal Blue is a little darker, more muted.
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 Blue on the colour wheel
All matches: Royal Blue equivalents · Blue equivalents