Semi-permanent dyes last three to four months. Permanent dyes, on the other hand, don't fade, but after a month or two, the roots, with your natural color, will become obvious. Unless you don't mind this two-tone effect, you'll have to dye your hair repeatedly.
半永久染只能維持二、三個月，永久染雖然不會褪色，但麻煩的是，一、二個月後頭髮長了，染過的和新長的頭髮便呈現兩截不同顏色，除非不在意，否則只好一再重複染髮。