See the photo below to discover what a difference grout choices can make.  Each of the four tile sections is identical, with the obvious exception of the grout color.  White grout (top) can make tiles seem more spread apart and allow the colors stand alone.  Black grout (bottom) draws everything together and makes bright colors pop.  However, dark colors can get a little lost in black grout. 

The middle sections illustrate how more neutral choices can usually be the safest choices when grouting.  It all depends on the project though.  Do you have bright colors that will be set off beautifully by black grout?  Then be bold, and grout with black!  Do you want more contrast and separation between the objects in your mosaic?  White may be the right choice then.  For all else...gray is my personal favorite.