The following fascinating account is related in the Darkei Teshuva.
There was a rav whose half of the hair on his head, half of his beard and one of his payos turned white, while the other half of these three remained black. Understandably, he was embarrassed in the presence of those who saw him as they would steer at him in astonishment. This rav asked the Shaul Umaishiv if it was permitted for him to dye his white hairs black or if it was prohibited because of the prohibition of לא ילבש גבר שמלת אשה, a man shall not wear a woman’s garment.
He replied that it was permitted to dye them for several reasons. The main foundation for this heter was due to the fact that the prohibition according to most authorities is rabbinical, and as such one can be lenient.
R' Yoel Ashkenazi was also asked this and he cites the words of the Shaul Umashiv who permitted dying the white hairs black. However, R' Ashkenazi rejects this and says that לא ילבש is a biblical prohibition and as such one cannot be lenient. He suggests dyeing the black hairs white so that this way it will all be white and he will be saved from any embarrassment.
Let us conclude with a similar question that was asked to the Divrei Chaim. A person had half of his beard turn white as well as one of his eyebrows. The other half of his beard and other eyebrow remained black. The question posed was if the man can dye the hairs black. The Divrei Chaim responded that it is biblically prohibited to dye it because of לא ילבש and therefore one cannot be lenient. He advises to dye the black hairs white because with that there would be no prohibition.
