I would argue with that. That would, to me, lead to the concept that an open trumpet is the only way to go even if it is a bad trumpet sound. No more mutes, no more electronics. No more Cootie, Dizzy, Miles, Don Ellis.... No more La Mer, Rite of Spring, no more Arutunian Trumpet Concerto..... An unmuted trumpet is already going to sound like a trumpet. A skilled player, though, can mimic the "ping" of a triangle, emote like a woodwind, slash like strings.... Something I used to do when getting bored playing the same stupid exercises over and over (that was back in the day before "fun" is what was taught) was as follows: I'd try to emulate the styles of kids I knew. "I'm going to play this like Bobby would, Billy and (yuk-yuk-yuk) Suzie." Later it became Herb Alpert, Maynard and some "legit" guys. Then it moved to oboes, strings, contra-bassons, car horns.... That sort of goofing around taught me tons about color and articulation without codifying it. I have a default sound, sure, but can modify it pretty much as I will. This kind of director's advice "play it like an oboe" is, I think, a great approach to break a player of a one-dimensional sound.