ΔE 17.3, loose: different enough that you should pick on purpose. Deep Amethyst is much more muted, cooler than Purple Tone. They are different kinds of paint: Army Painter Purple Tone is a wash/shade and Reaper Deep Amethyst is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Deep Amethyst is much more muted, cooler than Purple Tone. They are different kinds of paint.
The same difference applies the other way round: Purple Tone is much more saturated, 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.
See Purple Tone on the colour wheel See Deep Amethyst on the colour wheel
All matches: Purple Tone equivalents · Deep Amethyst equivalents