When you have removed an unwanted dark hair colour, only to find the hair slowly darkens again over hours or overnight, this is indicating the (removed) hair colour is re-oxidising. Re-oxidation occurs when oxygen molecules (either atmospheric or in applied hydrogen peroxide) make contact with artificial colour molecules. This process is how the artificial permanent colour produced in the first instance. However, if you have removed an artificial colour and then experience re-oxidation, it means those reduced synthetic colour molecules did not effectively flush from the hair during the rinsing stage.
How to remedy
You will need to apply Decolour Remover again. However, when you get to the rinsing phase, make sure you rinse for at least 5 minutes under comfortable hot, steaming water. Steaming water ensures the hair cuticle raises open, enabling the reduced hair colour molecules to flush out thoroughly. To be doubly sure re-oxidation does not occur again, rinse for 5 minutes, then wrap hair in a towel and wait 30 minutes to an hour, then go back and rinse again for a further 5 minutes. By allowing the moisture inside the hair to evaporate a little, then returning to rinse again, there is far more chance of removing any stubborn artificial colour molecules.
Re-oxidation can be an entirely random phenomenon. Occasionally, a colourant shade appears on the market that can be prone to re-oxidation after removal. Also, naturally, Auburn hair tends to re-oxidise after the removal of darker colours because Auburn hair has a high sulphur content, which can prove more resistant to reduction agents found in hair colour removers and perms. For these reasons, always purchase two colour removers, to ensure a second is at hand if the first application does evoke re-oxidation.