Ironbreaker vs Sparkling Amethyst

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Sparkling Amethyst is a little lighter, much more saturated than Ironbreaker.

Can I use Sparkling Amethyst instead of Ironbreaker?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Sparkling Amethyst is a little lighter, much more saturated than Ironbreaker.

Can I use Ironbreaker instead of Sparkling Amethyst?

The same difference applies the other way round: Ironbreaker is a little darker, 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.