Unlike temporary hair color that coats the hair shaft, semi-permanent hair dye uses lightweight compounds that penetrate the outer layers of the cuticle. As the dye stays near the surface of the strand, it usually fades within 12 washes – although darker colors like blue and black may last longer. The final color depends on your natural shade and hair porosity, resulting in a natural, shimmering look. Although you can still go for wild color if you want to make a statement – like hot neon pink.
Semi-permanent hair dye is ideal for trying a new color without committing. As there’s no ammonia or hydrogen peroxide, it won’t open the hair shaft and is gentle on your hair – but this also means your hair can’t go lighter.
Demi-permanent hair dye is half-way between semi-permanent and permanent hair dye. It doesn’t contain any ammonia, but the developer has a small amount of hydrogen peroxide. The ammonia is replaced by sodium carbonate or ethanolamine. These coloring agents remove less melanin, so you can’t take your hair as light. However, they do penetrate the outer cuticle, meaning the dye lasts longer – usually 20 to 28 washes.
