Sir Coates Silver vs Amethyst Purple

How close are they?

ΔE 42.2, loose: different enough that you should pick on purpose. Amethyst Purple is a little lighter, much more saturated than Sir Coates Silver.

Can I use Amethyst Purple instead of Sir Coates Silver?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Amethyst Purple is a little lighter, much more saturated than Sir Coates Silver.

Can I use Sir Coates Silver instead of Amethyst Purple?

The same difference applies the other way round: Sir Coates Silver 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.