It is odd that this question was posted when it was. I actually purchased 4 of these mics a while back for toms and guitar. On Thursday of last week I sent an email to Sennheiser asking what the diff was (between all black and the silver face) and this was their response;
The Black version have a flatter response across the frequency range. Which is really good for the application of guitars. The Silver version was a newer model released which has a boost in the higher frequency range around 5 to 8 khz, which is really bright.
It is basically two different capsule types with different sound characteristics. Our new model the E906, combines the two types with a switch.
Let me know if you have any other questions.
860-434-9190 EXT. 182 email@example.com
Hope that helps , cheers!