Holly Ive been waiting to see if someone answered you-I was clueless as to what you meant. Do you have a pic you could show us maybe?

Here's an article I wrote that has a picture of a picture frame I made. Open the article and then click on the photo to make it larger and you can see that the grout is randomly sponged with different colors, copper, bronze, and green onto a pale brown grout. I use acrylic paints you can buy at Michaels or Hobby Lobby; the brand I like is Ceramcoat. I use only natural sponges, also called sea sponges or cosmetic sponges. I wet the sponge first and squeeze all the water out, pour a little paint into a lid, dab the sponge into the paint and then onto the mosaic (all over, not just on the grout). Then I carefully scrub the paint off the tesserae while leaving it on the grout. Rinse your sponge out often; do not let the acrylic paint dry into it, and when you're done, rinse well.

Here is a link that might be useful: a Hubpage article

Silvamae .... I love your mirror ! It's hard for me to see the different colors of grout . But -- that's the point .
Subtle differences. Wonderful idea ! I'm glad Holly-bee asked !

*Sigh* Yet another idea to try .....