What is the ruling on using black shampoo to conceal gray hair for adornment?

If the shampoo dyes hair black or causes it to blacken, then its use is Islamically forbidden. Shampoo that darkens white hair and its effect lasts for two weeks is considered a dye. Sheikh Ibn Baz was asked about a mixture of henna and herbs that blackens hair, and he replied that there is no harm in using it to soften hair if there is no grey hair. However, with grey hair, it is not permissible to use anything that makes the grey hair black, due to the Prophet's (peace be upon him) saying: "Change this grey hair, but avoid black."