After the Roshkrish or opening of the icon, there are typically up to three sets of highlights and three floats over the skin and clothing. If there are background details those will also have highlights and floats, but the number will vary depending on the desired effect.
In this image of the Virgin of Guadalupe I actually added another layer to the Roshkrish because the colors had not dried as darkly as I wanted them. Then I started with the first highlights. On the face and hands the first set is usually the darkest. The process involves using three wells in a palette. In the first the pigment is mixed with egg only. In the second, water is added to some of the first egg mixture - usually about half and half. And in the last well there is only clean water. Areas where the highlight will be the strongest get the first mixture. The second mixture is then applied at the borders of the first and carried out a desired distance; finally with a clean brush, the edges of the previous application are touched with the clean water and carried to the outer border of the face so that ultimately only the underlying darkness will show through. Ideally, pigment flows through the water, settling itself in a relatively smooth gradation. Often you're not sure how it is going to dry - you apply the paint carefully as you can and then keep your fingers crossed that as it dries, the effect you were going for will have been realized.
I sometimes think as much praying goes into the making as the icon will ever experience once it is completed. There are so many steps where things can go wrong. Whenever my teacher hears someone suddenly gasp or suck in their breath, she is quick to say "Stop! Let it dry. It can be fixed." I hear her voice in my daily life now when I make other mistakes. It's a good mantra to have.
Highlights on garments are applied in a particular sequence, too. The first set of highlights are to define the physical body. You generally choose a color close to the underlying color but with a lot white added to it. And the three wells are utilized again. The second set of highlights represents the spiritual body.
Each time the highlights have dried thoroughly, a transparent float is applied - using pigment in a 1 to 5 egg/water mixture. Mixing 3 colors provides for a rich float.
I love mixing colors. It provides an ongoing education and constant surprise. Dry pigments may be very different looking once they're introduced to a medium. Some are strong, some are weak. Some need more grinding or they keep showing up in particles. Some can't be used till the final float because they keep coming through every succeeding layer. Some pigments are from heavy metals and need special precaution in using them.
Since the best way to learn these qualities is by experimenting with them, I've often come up with results that were disappointing. And then I use Olga's advice. I let it dry. I mix up another float. I may re - highlight an area. I fix it.