What you need to know before you dye

• First of all, the dyes will not be visible in black or dark hair, so bleaching hair to a pale blonde prior dyeing is recommended for best color results. The lighter the hair, the brighter the color effects of the dyes. The darker (or browner) the hair, the more subtle the color effects.
  
• Because these dyes are not permanent, the color fades every time you shampoo your hair. Dye effects last anywhere between 2-6 weeks, maybe even longer with proper care.
• Also called "vegetable dyes," they are generally safe to use because they're made of natural ingredients. None contain harsh chemicals like ammonia and peroxide normally found in most permanent hair dyes. It's PPD-free. So it's less likely to cause allergic reactions. Learn more about PPD.
• Ready to use. No need to mix with other agents or developers. Apply directly on dry hair.
  

Rule #1: Hair must be lighter in color than the dye

Semi-permanent dyes work very differently from the common boxed hair dye. Dyes like Manic Panic, Special Effects, and Punky will typically not be visible on black or dark hair, so you need to lighten or bleach your hair before you start using the dyes.

Rule #2: Color of hair and dye will mix.

Semi-permanent dyes actually work by staining hair; not by painting it. Thus, the lighter the color of your hair, the brighter the color result of the dye. The darker the color of your hair, the more subtle the dye effect. Refer to the color guide (see image) to know how light you should bring your hair before dyeing.

Also, if you have colored your hair before - be it copper red, strawberry blonde, or burgundy - your current hair color will still mix with the dye result. So for example, if you have reddish hair and use a blue dye, you probably will end up with a purplish result. So if you don't want to risk any unexpected results, remove previous colors before dyeing with the semi-permanent dyes.