The compliment about a new haircut arrives after the person has already lived through the uncertainty.

Praise usually turns up after the difficult part is over. Between the decision, the chair and the mirror at home, somebody has already gone through wondering whether it was a mistake, and by the time you say you like it they have made their peace either way. That does not make the compliment worthless, it makes it late. The support that would have mattered most was available earlier and quietly.

Say the kind thing anyway, and make it more useful than approval: that it suits them, or simply that you noticed. When someone tells you beforehand they are considering a change, that is when your opinion actually helps.