If you are looking for dramatic results or are changing your hair colour drastically, leave Wella Color Charm on your hair for the full 45 minutes according to the manufacturer's instructions. Leaving colour dye on your hair for longer than 45 minutes will not alter the colour results, but may unnecessarily damage hair.
Slight Color Change
If you are altering your current colour by one or two shades (either lighter or darker), you may achieve desired results by leaving Color Charm on your hair for 30 minutes. If your hair is dry or damaged, the dye will be absorbed by the hair follicles more quickly than healthy hair and 30 minutes of processing should be sufficient even for drastic colour changes.
Bottom Line
According to the manufacturer's instructions, Wella Color Charm can be left on hair for 30 to 45 minutes. The amount of time you choose to leave Color Charm on your hair depends on your desired colour results.