1.

1. Landmarks only for the pattern but I'm looking at a photo of the original mosaic.
Set the eyes ensuring that they look in the correct direction.

2.

2. I'm having to use some 5mm tesserae for the detail but this is not ideal and I get back to 10mm as soon as I can.
Eyes and line of the nose set.

3.

3. Mouth added, dark red line on the right. Line of background tesserae set to hold the smaller dark red in place. Complete the neck using full size tesserae

4.

4. Set the forehead and outline (his) right eye more.

5.

4. Set the lighter area of the cheek. Add the outline of his ear. Note I've left gaps for the dark brown of his hair to break up the forehead otherwise you get a very 'round' face.

6.

6. Set the full outline of his face, make sure that the tesserae are set in flowing lines to match the light/anatomy.

7.

7. More of his right side done, making sure that the lines go in the correct direction, here they must be down and in.

8.

8.Complete the last section.

9.

9.Add the hair and headdress. It's still difficult to see the finished piece until the rest of the background is put in.

10.

10. The finished face and head. The problem with this work is once again trying to use a combination of methods, ie mixing Template and Line copy. For the next one I will just mark out the eyes, nose, mouth and line of the jaw. Then I will ignore the original completely and set the face following my own style.
As I used only 2 colours for the body I deliberately didn't add in other shades to complete the face except fro the thin line of dark red. Unfortunately the light area beneath his right eye extends too far in a straight line giving him the look of having a mask on.
Colour matching, use of shading is a skill that comes with experience, what you must do first is to be able to set the tesserae in correctly flowing lines.