Royal Blue vs Arianrhod Blue

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Arianrhod Blue is a little darker, warmer than Royal Blue.

Can I use Arianrhod Blue instead of Royal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Arianrhod Blue is a little darker, warmer than Royal Blue.

Can I use Royal Blue instead of Arianrhod Blue?

The same difference applies the other way round: Royal Blue is a little lighter, 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.