Make improvements of her look

Example (Neg): "You would look much better, if you had red shoes, black ones to a black dress are a bit boring."

This is a dangerous one, because she could feel insulted very quickly, so you have to calibrate what kind of girl she is. If you say it like this it's kind of a neg, so it's only if she has a big bitchshield, if you transform the sentence you can make it much softer.

Example (Compliment): "You would look much better, if you had red shoes, they would fit perfectly to your black dress." or "I love your look, it's really pretty, BUT if you had red shoes, it would be perfect."

This gives her the good feeling of a compliment and that somebody really recognized the effort she put in her outfit. You show not only a sense in fashion and style, but also that you have standards and qualify the woman you meet.