What was different about Chroma the Great’s instruments?

His instruments did not make music, but rather, provided imagery..... colors.

"I don't hear any music," said Milo.
"That's right," said Alec; "you don't listen to this concert—you watch it. Now, pay attention."
As the conductor waved his arms, he molded the airmlike handfuls of soft clay, and the musicians carefully followed his every direction.
"What are they playing?" asked Tock, looking up inquisitively at Alec.
"The sunset, of course. They play it every evening, about this time."
"They do?" said Milo quizzically.
"Naturally," answered Alec; "and they also play morning, noon, and night, when, of course, it's morning, noon, or night. Why, there wouldn't be any color in the world unless they played it. Each instrument plays a different one," he explained, "and depending, of course, on what season it is and how the weather's to be, the conductor chooses his score and directs the day. But watch: the sun has almost set, and in a moment you can ask Chroma himself."