I finally got to bring the bright red out. Yesterday I laid the first line of the Red tesserae. These mark the top of Mariam's veil.
I also marked the rims of her eyes with black granite, and added to the sand on the side.
Once you have a few uninterrupted lines of one color it becomes easier to work simultaneously on several areas of the mosaic, because you can use this line to glue other tesserae against it.