Concept: Fire Lords traditionally dress like the stages of the sun to match their reign.

When Zuko is first crowned, he dresses in the colours of dawn. Bright yellows and pinks and even purples. He starts to wear more traditional crimson-and-gold robes after a few years, the colours of midday when Agni is at his strongest. And towards the end of his reign, when he's getting ready to pass the crown onto Izumi, he starts dressing like the setting sun.