Start by colouring the face; when you want to colour a large area evenly, try to avoid putting too much pressure on the pen nib and do not work too fast, as this will leave streaks.
Make sure there is a good ink flow so that it does not dry too quickly.
Top Tip: Sometimes a circular movement will work better to colour evenly.
Another layer of the same colour makes the area slightly darker, so this is good if you want to add subtle shadows.
If you add another layer of colour when the first layer is still wet, it will blur. To create a sharper edged shadow, wait until the first layer is dry before adding the second layer.