If you haven't sealed the grout yet, you could wet and only towel dry the tiled area leaving the grout dampened again, and then take any latex paint color you like and a small brush and create the effect.
The secret is to SUPER thin the color to make it more like a water color wash and clean wipe the tile edges as you go. Let the color soak into the water based grout until you achieve the desired hue and let dry for a few days. This is when you would want to re-clean the floor and add a grout sealer.
