Faces! Faces make the difference in artwork. For most of the iterations of my icon, I put off doing the faces since they are fussy and technical. There's a pretty good reason to do the faces first, though: if you're going to make big mistakes, make them at the beginning where you don't have to undo a large amount of established work.
The faces start off very pale and mask-like, not at all human. That's from laying light coloured paint layers on a dark form. Once the first cycle of paint is laid down, the second cycle is brighter and more colourful since light colours are being painted on light colours. You need the dark form, though, to create shadows.
Originally, the figures had a fairly realistic flesh tone. That looked horrible, since it broke the colour palette of the layout. And which flesh tone is realistic? Galileans likely would not have the same skin colour as comedian John Cleese, whose skin seems to match the look of most religious images. There are dark-skinned post-modern icons, but orthodox icons have yellow ochre skin since yellow ochre would have been a common pigment available to the medieval artists. You work with what you have.