How to compliment a girl on her hair?

Anonymous
She has the prettiest hair I have ever seen. The issue is that I have only seen her with her hair down once and did not compliment her, big fail. But regardless her ponytails are gorgeous, I have never seen hair so luminance on a girl. So her hair usually looks the same which is great, but if I compliment her I am afraid that she will think it is insincere since it looks the same as it always does. how should I approach this?
How to compliment a girl on her hair?
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