* Hair dye doesn't change the natural pigment of your hair permanently. The dye coats the hair shaft or, in the case of permanent dye, penetrates it to deposit color.
* White hair is hair that has lost its pigment (melanin). This happens naturally as we age or sometimes due to certain medical conditions.
* Dye can fade over time. As the dye washes out or the hair grows, the original color or white hair underneath will become more visible. This can give the illusion that the dye turned your hair white, but it just exposed the existing white hair.
* Sometimes dye jobs go wrong. Incorrect applications, using bleach too often, or using the wrong products can damage hair, and that can sometimes *appear* to make hair lighter or less vibrant.
In short, hair dye colors the hair; it doesn't fundamentally change the hair's pigment-producing cells.