If you replace her hat with an helmet - like Combat Helmet, she will replace it. Then you must save and kill her. Take the helmet and you will see her hair - style and colour. That's the only way. However, you can load the previous save.
Or, give her a ballcap with glasses, and she'll put it on, then give her a regular pair of sunglasses and she'll switch to them without putting her hat back on. On the PC you can also remove it with the console.