I agree with Al that the mouthpiece can make a large difference [again, I cannot believe I just agreed with Al]; HOWEVER, you CAN still darken the sound by relaxing the embouchure. This can be done by lowering the lower jaw and opening up the airflow and RELAXING. You can get that flugel sound out of a trumpet by doing this.
In one of our rehearsals our pianist in the quintet challenged me to play a song I usually play on flugel on the trumpet but make it sound like the flugel. I accepted the challenge and pulled it off will using the above technique. I then challenged him to make his piano sound like a Hammond B3. He let me down.