Symbolism, Imagery, Allegory
Gaiman wittily sneaks in a joke about how the clothes don't always make the man, but in Tristran's case, they kind of do:
Tristran Thorn in crimson and canary was not the same man that Tristran Thorn in his overcoat and Sunday suit had been. There was a swagger to his steps, a jauntiness to his movements, that had not been there before. His chin went up instead of down, and there was a glint in his eye that he had not possessed when he had worn a bowler hat. (4.217)
Tristran's wardrobe change thus symbolizes his growing sense of confidence now that he's where he's meant be (in Faerie, and taking charge of his life). We see other instances of this, too, like when the witch-queen becomes young and beautiful, and puts on nice clothes instead of tattered ones. Outer appearances (like clothing) are thus used to reflect inner realities, like states of mind and senses of purpose. This isn't a major symbol, but definitely one to keep an eye out for to enhance your reading experience.