The person changing their appearance may be celebrating, recovering, experimenting, or simply bored.

A visible change carries no automatic story. A new cut, a colour, a shaved head, a different way of dressing: it might mark something good, something hard, an experiment or a passing whim, and the outside gives you no way to tell which. Assuming one explanation puts the other person in the position of having to correct you. Interest without a theory is the gentler approach.

Say you noticed and leave room for whatever it is. A plain observation works better than a compliment aimed at a reason you invented. If they want to explain they will, and if not, the noticing was still kind.