My sons are similar. For them, I know that it is partially discomfort with change, and when it comes specifically to changes in personal appearance, because they experience some "face blindness" and the changes make it more difficult to recognize the person.
One day I put my hair into two braids, instead of my usual ponytail, and when I went to pick my sons up from school, one of them refused to get in the car with me until after we had talked for quite some time. I had to assure him that I was still me, and it took awhile.
I have never dyed my hair, but I can imagine that it would elicit a similar response.