According to hair professionals, black hair can easily be dyed with a lighter shade. But when you already dyed your hair black, it will be difficult to just colour it over especially with a lighter colour. If you really need to dye over black hair dye, there are some things you should do to prepare your hair for another colour. Actually when you apply hair dye to your hair, it does not actually change the colour of your hair but instead, it adds to the existing colour.
One should know that when you apply another colour to your black hair dye, the black dye will merely absorb all other coloured dyes applied over it. You have removed first or lessen the amount of black dye on the hair before applying other colours. Black hair dye will become lighter over time, but in case you want to dye over your black-dyed hair immediately, you should speed up the fading process. These processes may take at least two days to do. First you have to wash out the black dye first with dye stripper but it may contain chemicals that can ruin your hair. There are various remedies you can apply such as anti-dandruff shampoos that can help to fade hair dyes faster. Just repeat the process after a few hours of shampooing and drying to hasten the fading of black dye before applying other colour.
You can also bleach your hair but be sure that the black dye you used was vegetable-derived. If you used mineral-based black dye, you should not consider bleaching your hair since it will only result to hair burn and hair breakage.