Royal Cloak vs Arcane Blue

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Arcane Blue is noticeably lighter, more muted than Royal Cloak.

Can I use Arcane Blue instead of Royal Cloak?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Arcane Blue is noticeably lighter, more muted than Royal Cloak.

Can I use Royal Cloak instead of Arcane Blue?

The same difference applies the other way round: Royal Cloak is noticeably darker, more saturated.

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.