i would just say thanks. I don't know I wouldn't really think much of it. people compliment my hair all the time so maybe that's why it doesn't have an effect on me. my hair is white so its pretty attention calling. although, if you kept going on and on about how great my hair looked, I would feel kind of odd like something is up. it happened once. I felt alittle nervous. I got the feeling that it wasn't about my hair anymore. oh I didn't mean to sound full of myself btw.
It's not really the greatest idea to give a girl a compliment outside of a relationship as such buttttt if you absolutely feel the need to then yeah sure go ahead and do it. If you're going to compliment her hair say something like "your hair looks really nice today (make sure you add the word today), if she's done something different with it, she'll be glad you noticed. Girls notice the small things, rather than the larger ones