Semi-permanent hair dye? Why is it called that? How does it work?
There are all different kinds of hair color products, permanent, semi-permanent, Demi, permanent temporary. They all work differently.
Permanent hair color uses a chemical process that alters the structure of your hair. It's mixed with developer and opens up the outer layer of hair, called the cuticle, to react with the inner cortex and replace your natural pigment with new color that bonds to the hair.
Now semi-permanent dye on the other hand, works basically like a stain. Semi-permanent dye deposits pigments that mostly sit on top of the hair and fade with each wash. Semi-permanent doesn't mean temporary. It is going to penetrate the cuticle a bit, especially if your hair has been lightened. So, especially depending on the color, some semi permanent dyes can last for 24 washes or more.
If you're looking to dye your hair a vivid color the reason pre-lightening is so important is because the color just sits on top of whatever your base is. I like to compare semi perm to using markers on paper. The closer to white the paper is, the truer to tone the marker is. Going to show up.