I know of no example of an Amethyst Dragon in any D&D fiction.I think they are better than Amerthyst Dragons which are a "traditional" nothing.
There are numerous examples of heroic Red Dragon and Red Dragon riders in the Forgotten Realms. Red Dragon riders have heroic parts in BG3 as well as in both the Azure Bonds novels and the Cadderly novels. One of the members of The Brotherhood of the Griffon rescues and then rides a Red Dragon for a time in the Brotherhood of the Griffon novels. A Red Dragon, Brimstone, played a heroic role in the Rage of Dragons novels. That is 5 examples off the top of my head, all in the forgotten realms.
Can you give me any examples of heroic Amethyst Dragon riders in the Forgotten Realms? Any at all in any D&D fiction on any world?
That's part of the point. Why rely on old lore about counter-intuitive red dragons, when you can use the relatively blank slate of amethyst dragons? (As @Paul Farquhar said, the key thing here is not the dragons per se, but the dragon riders.)