Altdorf Guard Blue vs Amethyst Rayne

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Amethyst Rayne is much more muted, warmer than Altdorf Guard Blue.

Can I use Amethyst Rayne instead of Altdorf Guard Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Amethyst Rayne is much more muted, warmer than Altdorf Guard Blue.

Can I use Altdorf Guard Blue instead of Amethyst Rayne?

The same difference applies the other way round: Altdorf Guard Blue is much more saturated, 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.