From The Practice of Tempera Painting:
"As nothing can be more distant than a sky, we may begin the discussion of painting procedure with a hypothetical sky, and assume, for the sake of argument, that what is wanted is an even gradation from a dark blue zenith to a white horizon..."
"Let use choose cobalt blue as our basic pigment. If necessary, we may make up a special mixture, turning it a little green with oxide of chromium, or a little violet, say with Indian Red; but in this case,we must put aside some of the mixed color as a separate pigment, and keep it until the painting is finished; for it may be needed again. We will then take three color cups, and into the first put a fairly large amount of the cobalt blue (pure or compounded), and into the third, a very small amount. We will add a small amount of white to the first cup, and a large amount to the third. When each of these mixtures is stirred, we shall have one cup containing a dark blue, and one containing a light blue. Let us put some of each of these into the middle color cup, and mix them together to produce an intermediate blue. Each cup should have its own brush, and each mixture must be tempered with egg for use."
Note: I used straight Cobalt Blue (no green or red) and Titanium White. These are the pigments prior to tempering with egg.
Enough quoting... Instructions next call for applying each mix in a band, with a bit of overlap.
An egg tempera rule is to never try to remove spilled water from a painted surface. Let it dry. Here is an example of the rule not followed. A little dab with a paper towel ripped off all layers down to the surface. This same destruction occurs when I paint over a still wet surface. I find this completely unintuitive as properly tempered paint applied layer after dry layer works very well, so how can all those properly applied and dried layers be torn off so easily? No matter, the results are clear.
Here is my first cut with a bit of gradation applied. The sky is taped off as I have plans for something below and a border pattern. After days of technique practice, it's time to cut loose a bit. :-)
Thompson, Daniel V. Jr. The Practice of Tempera Painting. New York. Dover Publications. 1962. (Yale University Press. 1936.) p. 105-6.