Shell Shocked vs Amethyst Purple

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Amethyst Purple is noticeably lighter, cooler than Shell Shocked.

Can I use Amethyst Purple instead of Shell Shocked?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Amethyst Purple is noticeably lighter, cooler than Shell Shocked.

Can I use Shell Shocked instead of Amethyst Purple?

The same difference applies the other way round: Shell Shocked is noticeably darker, 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.