This article caught my eye, as it gives some details about what drummer boy "regalia" would have been like.
Here's what it says about drummer boy uniforms:
Trying to outfit each soldier with the same uniform was a hard task at the beginning of the war. States that assembled men to fight may have just worn what they had available until the colonies got the money and materials to get uniforms to all soldiers. Once this happened, musicians would wear the opposite colored coat that a fighting soldier would wear. If a continental soldier fighting in the war had a blue coat with red cuffs, a musician would wear a red coat with blue cuffs. They did this so that they could be easily found by a commanding officer and on the battlefield, it told the enemy that they were not carrying any weapons and were not a threat
I didn't actually know this when I wrote about drummer boys in The Riddle of Penncroft Farm, many years ago!
“We need drummer boys, Geordie. Join us, as Owens here has done.” He threw these words over his shoulder as he strode from the room. Jealously, I glanced at Owens. How much I wanted to take up the drum—and how impossible that I do so!
* * *
When the applesauce was done, I started into the hut with the kettle. Just then, a shadowy form appeared behind me. “Geordie?” “Aye?” I was so startled, I nearly dropped the kettle “I’ve been ordered to show you about.” This time it was my jaw that nearly dropped to the floor. It was Ned Owens, still as pudgy as ever, though now dressed in the full regalia of a drummer boy. His nose wrinkled in disdain at the smell of sickness and smoke permeating the hospital hut. His disdain gave me back my tongue. “Ned Owens. How is it you look so stout in the midst of this near starvation?” “Let’s just say I know which side my bread is buttered on.”
Jensen, Dorothea. The Riddle of Penncroft Farm (Great Episodes) Houghton Mifflin Harcourt. Kindle Edition.