While the ringing of the metal that horns are most-often made of (there are a few guys making them out of wood) is what any coloration heard from them is attributed to, the dimensions of the horn itself is more responsible for the coloration. It's a very technical matter, but information about the effects of horn throats on sound is available for anyone willing to dig to find it.
But ALL loudspeakers are colored, in one way or another, and in varying degrees. We all pick the coloration we find least objectionable, and learn to live with it. Very much like a wife ;-) .