The Dragon Ball universe's mythology and cosmos have both been greatly expanded in Dragon Ball Super. As if having Gods of Destruction, Angels, and Supreme Kai in each of the 11 other universes wasn't complicated enough, the show decided to reintroduce time travel for its most intricate villain origin story to date. To put it mildly, Goku Black's appearance in Future Trunks' timeline is convoluted. The multiverse-hopping, body-swapping, time-traveling antagonist has probably caused many viewers to pause and pick up a pen and paper.
Future Bulma dubbed him "Goku Black." Because he resembles Goku, dresses in black, and is evil, the true identity of this villain is eventually revealed to be Zamasu, the Supreme Kai of Universe 10. But where did he come from, and why is he so similar to Goku? To understand the chain of events that led to Future Trunks' timeline being destroyed again, it's necessary to first understand Zamasu.
Zamasu from Dragon Ball Super: Who Is He?
A Supreme Kai's role in their assigned universe is to initiate life and then to merely track its development. Zamasu is from Universe 10, where he completed his training to take over as Supreme Kai. Zamasu, however, comes to hate the mortals he is tasked with protecting, viewing them as violent, self-centered, and repulsive stains on the beauty of life. This hatred even transforms into a hatred for the other gods who, by refusing to intervene, allow the mortals to rule the world. In the end, he devises "Project Zero Mortals," a scheme to exterminate all gods and mortals from the multiverse.
Despite having an admittedly compelling origin story for a villain, Zamasu's transformation from a disgruntled Kai to an evil clone of Goku required a lot of juggling. Beerus travels to Universe 10 and destroys Zamasu in the main timeline, thanks to knowledge gained from Future Trunks' arrival, thus sparing the series' main timeline from his plans. In an alternate future where Beerus does not destroy Zamasu, the evil Kai apprentice murders his master and steals his Time Ring before traveling to Universe Six, the home of the Super Dragon Balls.
In this new, alternate future, Zamasu uses the all-powerful Super Shenron to wish to swap bodies with that timeline's Goku. As a Supreme Kai, he is severely limited in terms of combat strength, explaining why he wanted Goku's Saiyan abilities, so that he may carry out his plan himself. Once Zamasu obtains Goku's body and power, he promptly slaughters Gohan, Goten and Chi-Chi before using the stolen Time Ring to travel to Future Trunks's timeline. There, he joins the Zamasu of that time to enact Project Zero Mortals. It is here that he becomes known as Goku Black, as Future Trunks and Bulma are unaware of his real identity.
What Causes Goku Black to Turn Pink?
All of this, however, does not explain why Goku Black turns pink. Zamasu's wish transferred his consciousness into Goku's body, making him a Kai and a Saiyan with access to all of Goku's Saiyan power. When mortals like Goku and Vegeta use god power to become Super Saiyans, their hair turns blue. However, because Zamasu is a god, when he uses god power to become a Super Saiyan, his hair turns pink, or Rosé, as he refers to it. Simply put, Super Saiyan Blue and Super Saiyan Rosé are the same form; the difference in color simply reflects whether the user is mortal or god.
Dragon Ball has always been a fan of evil Gokus, like the non-canon Turles or when Captain Ginyu also stole Goku's body. Super showed impressive determination to making another evil Goku, folding in the expanded series mythology as well as the return of Future Trunks for good measure. Unfortunately, most of Goku Black's time in the series was spent explaining how he came to be in the first place. Zamasu's plan probably would have worked if he didn't have to spend so much time explaining it to everyone.