And yet more from The Practice of Tempera Painting:
"If, however, the color lies upon a ground darker than itself, the ground, instead of reflecting light, or having no effect upon it, absorbs part of the light that the layer of paint transmits. The color appears darker; and, being composed of a suspension of finely divided particles, it takes on a cloudy, smoky quality which may be called "opalescence." This quality, to which, in nature, the sky owes its color, and mist or smoke against dark objects, their blueness, stands half-way between opacity and transparency."
Here are examples of light color over darker. Most of these have a glaze over the left hand side. With the exception of the top most block which used a green and bottom with a white, the glazings used yellow ocher. I seemed to find that color closest in hue provided the most pleasing results.
I'm working with more pigments these days (And even more on the way. Sinopia shipped today.) and finding that each pigment takes a different load of egg yolk. Some of my mixes have been on the eggy side. I know this because the finish is too glossy and drying time increases. It's pleasing to see a properly tempered pigment during application. I can actually watch a brushstroke dry within seconds, the light sheen dimming out.
Thompson, Daniel V. Jr. The Practice of Tempera Painting. New York. Dover Publications. 1962. (Yale University Press. 1936.) p. 101.