Is it permissible to use a cream that restores white hair to its natural color (black) if it works by stimulating the cells responsible for the color, given that it is not a dye?

It appears that this is a type of treatment unrelated to dyeing with black. The default ruling for things is permissibility, and that includes seeking treatment with any cause or remedy proven beneficial through experience, unless this medicine is forbidden for a Muslim to consume due to its impurity, or because it contains something impure, or because its use results in harm. Therefore, there is no harm in using this cream if its use does not lead to a forbidden act.