On a girl I like really wavy hair. I love natural read heads but any hair color in general works if it fits the face. Short hair can REALLY add serious cute points but at the same time long (maybe to the shoulder blades) can add sexiness to a woman.
In my eyes, medium (barely shoulder long) wavy-straight hair is the best length, then have it get flowy at the ends, and as for colour.. anything really, as long as you have that length, most colours look amazing, not even kidding..
As for my own preference in colours though, I like Light Brown - Jet Black hair, Blonde - White/Silver hair.
As for long hair, I think that's fine too, as long as it doesn't reach below your scapula..