Jedi with purple lightsabers tend to be more balanced and sometimes use darker aspects of the force (not in his case). If you ever played KOTOR purple was the color that dark Jedi used (then again that game isn’t canon). Many Jedi that have purple lightsabers journeyed to the dark side at a time but came back and became even more light side force users.
The main reason mace windu had a purple lightsaber was because Samuel L. Jackson wanted a different colored lightsaber XD (sorry it’s so long, I tend to go on and on about these things lol)
Samuel Jackson asked George Lucas if he could have a different color to stand out in the crowd of Jedi in the battle scenes. The SW fans took the whole "purple" as a mix of blue/red into a deep fan fiction.
A blue lightsaber means that you are very good at combat. The green lightsaber symbolizes a strong connection to the force. A red lightsaber is obtained by either a rare red kyber crystal or more commonly the siths hatred fear and anger makes the lightsaber red. A purple lightsaber symbolizes that you are a jedi but you have a connection to the dark side.