Although happening offscreen, Eleanor Jane, the mayor's daughter, calling out her parents for how they treated Sophia.
As a woman, Celie finally telling Albert off, and leaving for good.
Sophia confronting Celie after she found out Harpo got idea of beating to tame Sophia, from her.
Before she storms away she advises that: "You better bash Mister's head in and think about Heaven later!"
Albert telling off his father and hustling him off the farm. Albert, up to this point, has acted like a complete piece of shit for the entire story; he marries Celie essentially to have a live-in servant, then attempts to rape Nettie, drives Nettie out of the house after she fights him off, and spends the next few decades being an abusive dick to pretty much everyone in his life. And then we meet his father, who explains Albert's entire crap personality within five minutes of his first appearance: he's destroyed Albert's self esteem and confidence with years of nagging, condescending criticism so badly that Albert lashes out with violence and abuse in order to assert power and dominance to counter it. When he starts whispering in Albert's ear that what he really needs is another wife (which suggests Albert's marriage to Celie was also prompted by him, possibly even while Albert was still grieving for his first wife and still longing for Shug, who he disapproved of), Albert realizes this, and kicks him out. Arguably, it's this moment that truly marks his Heel-Face Turn, and starts him on the road to recovery.
The song "Hell No," to the point where that was the musical's song showcased at the Tony Awards. (For added bonus, it's Sofia teaching Celie how to have her ownMoment Of Awesome.)