Here's a breakdown:
* How semi-permanent dye works: Semi-permanent dye coats the outside of the hair shaft with color. It doesn't contain ammonia or peroxide, which are needed to open the hair cuticle and deposit color permanently.
* Why natural color isn't significantly affected: Since the dye just sits on the surface, it doesn't lift or lighten your natural color. However, it *can* subtly change the tone, especially if you're going darker.
* Fading: Because the dye isn't deeply embedded, it washes out over time, typically lasting 6-8 washes. As it fades, your natural color will gradually reappear.
* Porous Hair: If your hair is damaged, bleached, or very porous, it might absorb a bit more of the semi-permanent dye. This *could* lead to a slight, temporary alteration of your natural color, even after the dye has mostly faded.
* Going lighter: Semi-permanent dyes cannot lighten your hair. If you use a shade lighter than your natural hair color, there will be little to no visible difference.
In summary: Semi-permanent dye primarily adds color to the surface of your hair and washes out over time, with a minimal impact on your underlying natural color. It is a great way to enhance or experiment with your hair color without a long-term commitment.