Why is the purple wedding called purple wedding?

I would suggest a few reasons for this. Firstly, and most obviously, when people choke they turn a shade of red initially, then as they asphyxiate, and blood flow is restricted around the body, they turn a shade of blue. This transition from red to blue can result in a shade of purple and so can easily be linked to choking. Given the manner of Joffrey's demise, purple seems appropriate.

There are other reasons though related to the origin of the poison, as suggested by the wiki for the series:

The wedding of Joffrey and Margaery has been dubbed the Purple Wedding by fans for various reasons. The poison used to kill Joffrey is smuggled to the wedding in the purple amethyst hairnet of Sansa Stark, while the wine the king drinks is described first as dark red and soon after as purple. Purple is also a color often associated with royalty.

Fans of Game of Thrones have dubbed King Joffrey's wedding and (SPOILER) death, as The Purple Wedding. It is not a name that is canon. The death of King Joffrey had nothing to do with Sansa. She was given an amethyst necklace by Ser Dantos Hollard, to wear to the wedding. The necklace held the poison in one of it's crystals. Sansa was completely unaware that the necklace she wore held the poison. In "A Storm of Swords," Sansa wears the hairnet that carries the poison to the feast. S4 E2 of "Game of Thrones," the series, has Sansa weaaring a necklace made of amethyst. That is the difference between books and show.

Lady Olenna Redwyne; The Queen of Thornes, played with Sansa's hair and necklace before the feast. After she did so, one of the crystals was missing. The poison inside the stone made its way into Joffrey's wine goblet when Tyrion picked it up and put it on the table, right next to Lady Olenna and the King.

The Red Wedding was called as such by George R.R. Martin, who has said many times that he based it on an event in our own history. It comes from an event in Scotland, called the Black Dinner. Martin has also said in various interviews, that in Dark Ages hospitality laws were taken very seriously. If you broke bread even with your enemy, it was not the time to attack or fight. It was a time of peace by law. By violating that law, "they condemn themselves for all time."

We've since seen evidence of this in the books:

with the Brotherhood Without Banners, who are now led by a certain Lady. They have been killing Freys all over the North, as well as anyone who had wronged House Stark.

The Purple Wedding has much the same meaning and consequence. It was a political move, made by Littlefinger and The Queen of Thornes. We will see if they have condemned themselves and all they stand for, respectively, as well. It seems Lady Olenna has already begun to suffer the consequences with what is happening to her family at the hand of Cercei Lannister.

It's a reasonable name given to an event similar to the Red Wedding, Both weddings ended in regicide. King Joffrey Baratheon sat the Iron Throne at the time of the Purple Wedding, and Robb Stark was King in the North when he was murdered at the Red Wedding. It is simply an easier name used to refer to Joffrey's death by fans.

I believe that the term "Blue wedding" is sometimes used to describe a great/lucky wedding (probably coming from the quote "Something old, something new, something borrowed, something blue")

And a "Red wedding" is usually one containing blood and/or murder.

So you combine 'Blue + Red' and get 'Purple'.

The series already had a very prominent "Red Wedding", and the death of Joffrey was considered a great/lucky thing for the realm, it seemed logical to combine the two to come up with "Purple Wedding".

Because the hairnet that Sansa wears is coated in amethysts which are traditionally purple. It is the gem that causes the strife.

The necklace of Sansa has purple beads and that is what creates a big turn of events in the story. Maybe that is why it is known as the purple wedding.