It's common knowledge among hair professionals that black hair can easily be dyed a lighter shade or color. But once you've already dyed hair black, it is difficult to simply color over, particularly if you want a lighter color or shade. If you absolutely need to dye over black hair dye, then there are a few things you need to do to prepare your hair. Applying hair dye does not actually change the color of your hair, but rather add to the existing color.
Once you've applied black dye over your hair, the black dye will simply absorb all other colored dyes applied over it. This means you first have to remove or at least reduce the amount of black dye on the hair before applying other colors. The black dye will fade over time, but if you are in a hurry to dye over your black-dyed hair, you would have to hasten the fading process. Any of these tips would usually take at least two days to do.
Wash out the black dye first. Dye strippers might contain harsh chemicals that can harm your hair. But there are several remedies you can apply, which involve using over-the-counter products. For instance, anti-dandruff shampoos are known to quicken the fading of hair dyes. These usually contain conditioners and are fairly safe and gentle to hair. You can hasten the fading of black dye by shampooing, drying, and then repeating the process after a few hours.
You can also use clarifying shampoo. Any shampoo labeled as including acetic acid in its ingredients will do. Another alternative is applying a hot oil product. Be sure to thoroughly wash your hair after any treatment involving hot oil. You will notice that your hair might still be black, but it will no longer be as shiny as before, since dye has been washed off. This way, your hair can already take in other color dyes.
Depending on the black dye you originally used, you might want to bleach your hair first. This can only be done if your original black dye was vegetable-derived. Since bleach will affect both the dye and your natural hair color, this will usually result in a drastically different color (like orange or red, for instance). But you can more easily dye over this. If your black dye is mineral-based, however, you should avoid bleaching at all costs, as this will only make your hair burn and break apart.
Apply the new color. Once you have washed off most of the black dye, the next step is to re-color your hair. This might be a tricky step, because even as you've already washed most of the black dye off, there is still be some residue left. Therefore the trick here is to apply a dye that's three to four shades lighter than the color you're aiming for. This way, even if there is still some black dye residue left on your hair, you can get an approximation of your desired color.
Remember that dyeing over black hair involves removing or reducing the amount of black dye first. The intermediate result might not always be ideal (unless you really want bright orange hair), but it's a necessary step.