I read nothing about coloring your hair, only that the employees had to be well groomed with basic standards that are normal for most people. She sounds like someone who wanted to cause an accident. She succeeded. I hope she pays every penny in court costs.
Personal best would cover coloring your gray hair. I don't think she was one to cause a fuss, because they employed her for 21 years. If you were in a situation where your employer decided to mandate changes that you weren't comfortable with (I have no examples to cite-just use your imagination), wouldn't you kick up a fuss? Me? I'm an asshole and I'd show up looking like Zsa Zsa Gabor. Hey, they told me to wear makeup.
Seriously, the fair thing would be to "grandfather" the established employees who give good service, while holding the new hires to the new standards. 21 yrs=4 yrs until retirement? Hmmmm....