ΔE 16.8, loose: different enough that you should pick on purpose. Sparkling Amethyst is much more saturated than Plate Mail Metal.
Can I use Sparkling Amethyst instead of Plate Mail Metal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Sparkling Amethyst is much more saturated than Plate Mail Metal.
Can I use Plate Mail Metal instead of Sparkling Amethyst?
The same difference applies the other way round: Plate Mail Metal is much 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.