Another time I will plan this in advance and make a more definite border to hold the grout.
I didn't want to do a huge area with the black grout powder so here is what my experiment looked like. It's hard to tell in this small shot, but it was looking too busy for me along with the edges done in white.
In this photo I tried the white powder and feel the overall affect is better. It keeps the body of the mirror more solid and the edges more "lacey". The dishes have enough colors to carry off the white in my opinion.