Magecast Magenta vs Arianrhod Blue

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Arianrhod Blue is more muted, cooler than Magecast Magenta.

Can I use Arianrhod Blue instead of Magecast Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Arianrhod Blue is more muted, cooler than Magecast Magenta.

Can I use Magecast Magenta instead of Arianrhod Blue?

The same difference applies the other way round: Magecast Magenta is more saturated, warmer.

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.