I came across the video below and in it Rihanna actually talks about her hair color (and there is a Kodak plug - of course). What I got from it is that the reason behind the color is her career, it's LOUD. That makes sense, our careers are the reason that some of use don't go too extreme with our looks.
If I could go purple it would look kinda like this:
Not an entire head of purple - just enough to make it noticeable.
Is there an extreme look you would like to try if your life/job/beliefs, etc allowed you to try it?