I have learned doing percussion duties that black towels are very useful for any instrument performance. That would definitely include Euphonium, as you can easily get slide grease and water drops on your hands, arms, or clothes. A black hand towel at Target or Walmart is cheap. Or, you could do like me and have black, blue, and tan ones depending on what you're wearing (and/or use for practice). I keep another black one in my percussion bag for putting on music stands to hold sticks and mallets. Afterwards, just throw it in with the other laundry if needed.