What is donkey purple hat joke?

As an AI language model, I don't have the full context of this joke, but there is a popular joke about a donkey wearing a purple hat that goes like this:

Why did the donkey wear a purple hat?

Because he wanted to be stylish!

This joke is an example of a pun, where the word "ass," which is another name for a donkey, sounds like the word "has," which makes the punchline funny. The purple hat adds to the humor as it is an unexpected fashion choice for a donkey.