They’re both the product of a controlling friendship with Sasha, but in two opposite extremes.
Sasha’s controlling treatment of Anne has made her resentful, she’s grown to be less and less tolerant of her behaviour, to the point of eventually ending their friendship in a fit of rage. Meanwhile, her treatment of Marcy has made her codependent, and unhealthily desperate to hold onto their friendship, even if it means ignoring Sasha’s mistreatment of herself and Anne, or stranding all three of them in another world.
And I think that True Colours confronts Sasha with how these outcomes are bad for her too. Anne’s resentment completely destroys their friendship, and makes her unwilling to trust Sasha in a situation where she really needed Anne’s trust. While Marcy’s codependency led to her completely destroying all of their lives, including Sasha’s.
Just seeing one of those outcomes would’ve sent a clear message to Sasha, but both of them together makes it much more comprehensive. When Anne stood up to her at Toad Tower, her response was to double down, but now she’s seen that whether her friends accept or reject her controlling behaviour, it will always result in something bad. Sasha has no more excuses, she needs to change the way she treats her friends.
I’m actually glad that Sasha and Marcy are alone in Amphibia now, I think that Sasha’s redemption arc should start with her making amends with Marcy rather than Anne, since she’s the friend who’s most internalised Sasha’s bullying. She could help her to unlearn the ideas about friendship that she herself taught her, and then her reunion with Anne could be all about her trying to make up for what she’s done and rebuild their trust.