Dyeing some parts of the hair only

If the hair is dyed black, the Prophet (peace and
blessings of Allaah be upon him) forbade that, when he ordered changing
white hair but avoiding black. He said: “Change this white hair but avoid
black.” 

There is also a stern warning against doing that, which
indicates that it is haraam to change white hair by dyeing it black. As for
changing it by dyeing it other colours, the basic principle is that it is
permissible, unless it is done in the manner of kaafir women or immoral
women, in which case it is haraam for that reason, because the Prophet
(peace and blessings of Allaah be upon him) said: “Whoever imitates a people
is one of them.” End quote.