Purple Interference vs Amethyst Purple

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Amethyst Purple is a little darker, more muted, warmer than Purple Interference.

Can I use Amethyst Purple instead of Purple Interference?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Amethyst Purple is a little darker, more muted, warmer than Purple Interference.

Can I use Purple Interference instead of Amethyst Purple?

The same difference applies the other way round: Purple Interference is a little lighter, 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.