I don’t recall Amethyst ever inverting her color palette, unless you’re referring to the cat. In which case I wouldn’t consider that a palette swap since when she transforms into something with hair it’s always light, and since a cat is technically covered in hair it only makes sense that form would be too - if you shaved her in that form you’d probably see her dark skin underneath.
And I’m not sure I’d buy that PD just inverted her hair and skin since it’s not a matter of hue so much that one glance tells you that they’re two completely different colors. There’s also several dark shades of pink in PD’s design that past examples of Gem shapeshifting should dictate would make up some of the layers of her dress, but are no where to be found on Rose’s design.
And yeah it’s frustrating how many times Steven’s pulled a new power out of his ass, or the number of disrepencies get hand-waved as a “diamond powers.” It feels like the writers often make the rookie mistake where they’ve already written everything up in their head so they already know all these details when writing scenes, but then forget the audience can’t read their minds so there’s all these moments that feel contradictory or confusing from the perspective of a viewer.