If there is anything that Star Wars fan-favorite Mace Windu is known for, it is his purple lightsaber. The unique color of his weapon has set the Jedi master apart from the rest. Part of the appeal was Samuel L. Jackson's stories about the weapon. However, the character hasn't always brandished his purple lightsaber during battle.
Star Wars fans probably know the story all-too-well: After The Phantom Menace, Jackson approached George Lucas hoping to change the color of Mace Windu's lightsaber so that he will stand out from the sea of blue and green lightsabers during the Battle of Geonosis scene in Attack of the Clones. While Lucas initially wasn't initially crazy about the idea, he ultimately gave in to the actor's request.
Before that, however, it would appear that the Star Wars creator original plan was for Mace Windu to wield a blue lightsaber. In fact, this was what he appears to be carrying in the first Star Wars prequel movie. The weapon he had in The Phantom Menace had the same hilt design as the blue-bladed one that appeared in the 2000 four-arc comic book story Jedi Council: Acts of War.
While a lot of people do not mind the change since they are aware of the well-known story about it behind the scenes, there had to be an in-universe reason to make sense of the switch. And sure enough, it was provided in Legends.
In the comic story arc titled Emissaries to Malastare released in 1999 and set just a few months after the events of the first Star Wars prequel, it was revealed that Mace Windu took part in a Jedi tradition called Concordance of Fealty. Here, a Jedi entrusts his lightsaber to another member of the order. For an unspecified amount of time, they will use the lent lightsaber as their main weapon.
This was meant to help the participating Jedi to learn to trust each other and form a sacred bond. Mace Windu exchanged his purple lightsaber to Eeth Koth, who had a blue lightsaber.
This would explain why Mace was seen using this color in different iterations of his character before Attack of the Clones. It would also suggest that Jackson's request was made pretty early on that allowed the comic, released a few months prior to the filming of the second Star Wars prequel movie, can accommodate the change.