I'm retrying last nights English Ivy, this time with an easy, light touch. Many thin coats is the way to go. And, it really works! I'm going back in for additional depth and as for background, not sure yet.
From The Practice of Tempera Painting:
"The secret of ease and expedition in tempera painting may be summed up in one simple formula--Get your tempering right, keep your color liquid, and have your brush squeezed almost dry. In a word, make haste slowly. Tempera painting is as far from the technique of water color on the one hand as it is from the plastic paint-pushing of oil handling on the other. It is much closer to the technique of pencil rendering; and the tempera painter will do well to bear that comparison in mind, and handle his brush as if it were a pencil. When you want to lay a tone in tempera, do not try to float a wash, and do not try to spread the paint out with a brush; but run over the area to be covered with quick, easy strokes, like pencil marks. Follow this first coat with a second, applied in the same way, running the strokes perhaps at a little different angle, according to the form; and continue in this way until you have build up a deep and even a body of color as you want."
Thompson, Daniel V. Jr. The Practice of Tempera Painting. New York. Dover Publications. 1962. (Yale University Press. 1936.) pp. 99-100.