With permanent hair dyes, hydrogen peroxide is used to bleach the hair and then color is added. Semi-permanent dyes, on the other hand, are applied directly to the hair.
所謂永久性染髮，就是染髮劑中的雙氧水先將黑髮色素漂淡再染上顏色；半永久染髮，則是直接將顏色附加在原來的髮色上。