What is the key difference between permanent and semi-permanent hair color?

The key difference highlighted in the correct choice is that permanent hair color contains ammonia and requires a developer to achieve a long-lasting effect. This is significant because ammonia opens the hair cuticle, allowing the color to penetrate deeply and create a more permanent change in the hair's color. The use of a developer, which generally contains hydrogen peroxide, also facilitates the lifting of the natural hair pigment, allowing for a more versatile range of color results, including lifting and depositing color simultaneously.

In contrast, semi-permanent hair color typically does not contain ammonia and does not require a developer. Instead, it stains the outer layer of the hair and provides a temporary color change that typically lasts through several washes. This fundamental difference is crucial for clients who seek long-lasting results versus those looking for a more temporary change. The aspect of damage is also important; while any chemical process can cause stress to the hair, permanent colors generally have stronger formulations than semi-permanent options.