Is it permissible for a woman to change the color of her hair only for her husband and mahrams? Is hair dye forbidden or permissible? And does this fall under Allah's saying: "...and I will surely command them, and they will change the creation of Allah..."?

It is permissible for a woman to dye her hair with colors other than black. This does not fall under changing the creation of Allah mentioned in His saying, the Exalted: (and I will surely command them, and they will alter the creation of Allah). The predecessors interpreted this as castration, tattooing, or altering the religion of Allah and His natural disposition.