Paintings of this type, were often painted in the elaborate technique, using pigments mixed with hot or cold beeswax and other ingredients such as egg, resin, and linseed oil. This versatile medium allowed artists to create images that in many ways are akin to oil paintings in Western art. The boy’s head, for instance, stands out with an impression of real depth from the light olive-colored background. His face is modeled with flowing strokes of the brush and a subtle blend of light and dark colors. Shadows on the left side of the face, neck, and garment and bright shiny spots on the forehead and below the right eye indicate a strong source of light on the boy’s right. Most arresting are the eyes, dark brown with black pupils that reflect the light with bright spots. This manner of painting, which is very different from the traditional Egyptian style but was well known in Greco-Roman Egypt, originated in Classical Greece in the fifth and fourth centuries B.C.
Excerpts from My Greek Odyssey blog
Cf. more portraits at Wikimedia, and Protestant and Orthodox iconography compared