Here is a short tutorial how I painted my clone trooper faces.
1. Prime it with Corax White.
2. Apply some Contrast Guilliman Flesh followed by an layer of Contrast Skeleton Horde.
3. Painting the eyes with whatever black and white color you have available. Outline the eyes with some Model Color Saddle Brown.
4. First highlight for the flesh tones using Model Color Brown Sand.
5. Second highlight for the flesh tones using a 1:1 mixture of MC Brown Sand and Model Color Basic Skintone. Use black for the collar and the hair.
6. Details: Final highlight for the flesh tones using pure MC Basic Skintone. Carefully highlighting the hair and the collar with a lighter grey. Eyebrows have been a 1:1 mixture of black and MC Brown Sand.
Optional: The scar has been made of a broad line of MC Saddle Brown with a thinner line of a 1:3 mixture of a red color and MC Basic Skintone on top of the broader, darker line.