I was wondering why Miroku's robe (I don't know what's call) are purple.
I mean he is a buddhist monk and most of the time they are wearing red, yellow or orange, like in Avatar Last Air Bender.
I am no expert on buddhism so if anyone have an explanation??

Miroku actually wears the black robes of a Japanese Buddhist Monk. It's his Kesa or Kasaya, which is kind of a shawl draped over the robes as a reminder of the garments of the Buddha, that is purple. Traditionally, they are pieced together from various found/donated fabrics and don't have to be a particular color. Over the years, the kasaya became more "standardized", but back in the day, I guess it would have been common to see them in various colors.

Also, manga and anime are known for playing fast and loose with tradition. Miroku might have been depicted in purple as an artistic choice.

It might be because of how typical Japanese Buddhist monks' robes look. They differ by the country that practices Buddhism. According to a website that features the style of the robes, the color of the kashaya, which is the purple fabric wrapped around Miroku's robe, is just one of many colors that it comes in. It could come in red, yellow, orange, and possibly in purple in Miroku's case.