I am tiling my kitchen wall in a mosaic patern using mostly dark green tile with some white and beige tiles. I want the grout to be the same as the green tile. I found a grout (Polyblend non-sanded) in mallard green which by the color strip on the box looked as if it would match the tile; however, when it dried, it was much lighter. What can I add to it to get the correct color. Is there a pigment I can add that will not disrupt the chemical reaction of the grout?