Is it permissible to supplicate for the spread of white color throughout the body for someone afflicted with vitiligo, and is that considered altering Allah's creation, knowing that treatment has been attempted without success?

This supplication is not considered an alteration of Allah's creation, for change through supplication is an act of Allah Almighty, not an act of the servant. There is no transgression or discontent with Allah's decree in this supplication, as its essence is a request for healing, which is praiseworthy both religiously and intellectually.

However, it is better for the questioner to ask her Lord for well-being and healing without specifying a method for it. It is more complete to seek out and suffice with the supplications from the Quran and Sunnah. Among the comprehensive supplications for seeking healing is the supplication of Ayyub (Job), peace be upon him: "Indeed, adversity has touched me, and You are the most merciful of the merciful" (Quran 21:83), and the prophetic supplication: "Remove the suffering, O Lord of mankind, heal, for You are the Healer, there is no healing except Your healing, a healing that leaves no illness behind."