So here is a detail of one of the Archangels in a beginning stage. After the greenish brown sankir was applied, the drawing lines were reestablished and the facial highlights begun. The lighter skin colour looks patchy, but that was corrected later. Egg tempera can be a little unpredictable -- sometimes that's wonderful, at other times not so much. That is, for a beginner with only eight years under my belt it's unpredictable! But I'm getting a little more comfortable with it every year.
The red you see on the halo and the collar of the angel's robe is underpainting for the gold leaf. Gold leaf is so thin that it's transparent. You can affect the overall look of it by your choice of colour underneath. A good bright red results in a warmer gold once the leaf is applied.
I generally prefer to gild over bole (more on what that is later) -- if you know gilding at all you know why. But because I had not used bole for the Christ and Theotokos icons, I didn't use it for the saints or archangels. I can't remember why I didn't use bole to begin with; it must have been expedience, since gilding with oil size is much faster and easier.
Bole is a clay mixture which goes on wet, dries into a hard surface and provides a slightly raised effect to the gold -- very pleasing to the eye. It also makes a 'cushion' so that the gold can be burnished. Burnishing is basically polishing to a high shine. Up close I think it could be described as sort of 'mashing' the gold flatter so it is more reflective. There's my high tech description. Later I'll describe the whole bole-gilding process, at least as I know it (imperfectly). It's quite an involved thing in itself.