Bright imagery brings this story to life. Browning wrote this poem to amuse a young friend who was sick in bed, and the vivid pictures he paints seem designed to take a kid out of his uncomfortable situation and into a folktale.
For instance, Browning puts portraits of the Mayor and the Pied Piper right next to each other, creating pictures of the two men not just through description but through their contrast with each other. When readers meet him, the Mayor is sitting at his council table "looking little though wondrous fat," short but majestically round. The Piper, on the other hand, is:
[...] the strangest figure!
His queer long coat from heel to head
Was half of yellow and half of red;
And he himself was tall and thin,
With sharp blue eyes, each like a pin,
And light loose hair, yet swarthy skin,
No tuft on cheek nor beard on chin,
But lips where smiles went out and in —
These two men have a kind of Laurel-and-Hardy difference in physique, a comical contrast. Notice, too, how long the narrator spends describing the Piper compared to how long he spends describing the Mayor. The Mayor is just an everyday well-fed bureaucrat; the Piper, on the other hand, is a strange fellow like no one in this town has ever seen, and he gets described in as much detail as a newly discovered species of parrot. The image of his "lips where smiles went out and in," in particular, suggests he's a mysterious, cagey sort of guy: the smiles that go in sound like private smiles, smiles he doesn't share with the world at large.
There's a similar sense of mystery and magic in the lines where the Piper begins to play:
To blow the pipe his lips he wrinkled,
And green and blue his sharp eyes twinkled,
Like a candle-flame where salt is sprinkled;
Besides evoking an eerie, unexpected sparkle, this image suggests there's something a little dangerous about this man and his fiery look.
His piping produces this astonishing result:
And ere three shrill notes the pipe uttered,
You heard as if an army muttered;
And the muttering grew to a grumbling;
And the grumbling grew to a mighty rumbling;
And out of the houses the rats came tumbling.
Great rats, small rats, lean rats, brawny rats,
Brown rats, black rats, grey rats, tawny rats,
In this theatrical moment, the townspeople (and the readers) hear the rat stampede before they see it. When the rats finally put in an appearance, the description of their many shapes, sizes, and colors works alongside intense parallelism to suggest that there's every kind of rat here—but they're all still rats, zillions and zillions of rats, unified in their rattiness.
A similar movement from sound to sight also conjures up the moment when the Piper pipes again, this time in revenge:
There was a rustling, that seem'd like a bustling
Of merry crowds justling at pitching and hustling,
Small feet were pattering, wooden shoes clattering,
Little hands clapping, and little tongues chattering,
And, like fowls in a farm-yard when barley is scattering,
Out came the children running.
All the little boys and girls,
With rosy cheeks and flaxen curls,
And sparkling eyes and teeth like pearls,
Once more, a sound—this time, that of happy scrambling—arrives, ominously, before the children come into sight. When the kids finally appear, the speaker's imagery stresses their beauty and sweetness: they're all rosy-cheeked treasures, the worst thing in the world for their parents to lose.