* Hair Dye is Temporary: Hair dye, especially vibrant colors like purple, coats the hair shaft. It doesn't change the underlying pigment of your hair.
* Grey Hair is About Pigment Loss: Grey hair occurs when the cells that produce melanin (the pigment that gives your hair its color) slow down or stop producing melanin altogether.
* The Dye Fades: Over time, the purple dye will fade, revealing your natural hair color underneath (which may be grey if you have naturally grey hair).
In short: The purple dye fades, revealing whatever natural color is underneath.