ΔE 15.6, loose: different enough that you should pick on purpose. Amethyst Rayne is a little darker, warmer than Thunderous Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Amethyst Rayne is a little darker, warmer than Thunderous Blue.
The same difference applies the other way round: Thunderous Blue is a little lighter, 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.
See Thunderous Blue on the colour wheel See Amethyst Rayne on the colour wheel
All matches: Thunderous Blue equivalents · Amethyst Rayne equivalents