Why purple?

Why does Mace have a purple light saber?
I've heard many myths *giggled a bit... but any one know really why?

I saw a interview with Samuel J and jokingly he said it's so I know where I am and every else knows too.
I also heard it's from his sith and light knowledge.
I also heard
It's a special gem from a world he saved.
Can some one bring light to this lost soul. Lol thanks in advance for answering. I've always wondered.

Samuel L. Jackson asked for a purple Lightsaber. See http://screencrush.com/star-wars-samuel-l-jackson-purple-lightsaber/

Describing the climactic battle sequence in 2002's 'Star Wars: Episode II -- Attack of the Clones,' Jackson recalled:

We had this big arena, this fight scene with all these Jedi and they’re fightin’ or whatever. And I was like, well s---, I wanna be able to find myself in this big ol’ scene. So I said to George, "You think maybe I can get a purple lightsaber?"

Although Lucas pointed out that lightsabers traditionally only came in red or green, Jackson responded with some pretty powerful logic: "Yeah, but I want a purple one. I’m like the second baddest Jedi in the universe next to Yoda."

Ultimately, said Jackson, "When I came back to do reshoots, [Lucas] said, 'I want to show you something, this has already caused a s--- storm online.' And he had the purple lightsaber! And I was like yeah! And so I could find myself in that big fight scene. There's like 300 lightsabers, and I'm like, ‘There I am, right there.’"

Read More: Samuel L. Jackson on the Hilarious Origins of His Purple Lightsaber in 'Star Wars' | http://screencrush.com/star-wars-samuel-l-jackson-purple-lightsaber/?trackback=tsmclip

Is there a canon reason Mace has one "in universe"?

I'm not aware of a canon reason but I remember reading that his style of Lightsaber combat is called vapid which borders on using the Dark Side. He beat the poo out of Sidious unless Sidious let him win but I don't think he did. If there is a canon reason I'd love to hear it.

Me too!

I also like to think that Mace would have won had Anakin not showed up. Kid ruins everything. hahaha.
I genuinely think Palpy was counting on Anakin coming, but he was improvising with the lightning. But, once Anakin showed up, he put it all together and came up with a plan.

There are two reasons he beat Sidious. One was the style he created vapid which rides on a joy of battle that is a knife edge from being dark side rage. The second is he could see shatter points in the force seeing weaknesses or key moments in peoples fates. As for his saber color as far as I know it was a special crystal from his home world.

His home world was?

Bro John that's one story I read and George had to write it in. Thank you so very much!

I can chime in on the fictional or in-universe explanation!

In the "Legends" cannon, there's a ton of history about Lightsaber crystals. If anyone is curious and would like to nerd out, you can check it out right here.

As for the new canon though, 'Kyber Crystals' are the offical crystals that are used in lightsabers. They have NO color, until they are chosen by a Jedi. Most often a sort of vision / force quest (happens in Clone Wars) takes place, leading the Jedi to one.

The Force guided their selection, finding a crystal that matched them as Jedi. Crystals lacked color before they were chosen by a Jedi. Once chosen, most lightsabers became blue or green, though other shades were created in rare instances, most notably Mace Windu's purple blade, the yellow blades of the Jedi Temple Guards, the black darksaber and Ahsoka Tano's white lightsabers.

While I love the deep history of Legends, I really like the colorless crystals that change once they find a user.

I've heard that too! Mace was making one and he said it can be better so the council sent him somewhere planet and a crystal from that planet is said to give them tint.