The best way to darken your violin, if it is too bright, is to put strings on it that will darken the tone. I recommend Obligatos, Piastro Evahs, Corellis or Piastro Passione. These strings are on the warmer end of the spectrum and will even out a bright violin. If you have a warm violin, then you could consider Helicore, Vision or Vision Solo, or Kaplan Strings. Vision strings are very bright but rather complex, and I enjoy them despite their brightness.