When Yuria was taken away forcefully by Shin, her hair colour was still red. However, when she was being held captive, her hair colour changes to white. Why is this? Could she have been exposed to radiation?
What I get from the series is that she believed that Shin had killed Kenshiro and she is slowly being stressed by being with Shin. This is shown even in reality where stress causes grey hair.
So, in conclusion, I would say it's a stress thing and white was the chosen color. Either that or it lies in Shin's treatment of her which is pretty vague.
According to the wiki:
For reasons unknown, her hair color in the series changes from auburn prior to being captured by Shin, to pale violet when with Shin, and then golden when she returns as the Last Nanto General.
We cannot be sure what were the reasons behind this change!