I have been mixing a teaspoon of pigment with a few drops of water to make a paste. In a separate bowl I pierce an egg yolk to make sure all the eggy goodness goes into my bowl and not the egg yolk sack. I mix half of this yolk with my paste using a palette knife. You need to work it very thoroughly to push all the pigment into the egg. If you can, by a muller which is more effective at mixing the paint. The egg acts as a binder, if it is not properly mixed, your pigment will fall off and become unstable.
I have been mixing pearl lustre pigments (from Cornelissen's - http://www.cornelissen.com/) with my raw pigments to get a shiny surface. These pearlescent pigments are even more unstable, so I recommend adding more egg than usual. The paint is very quick drying - in a bout ten minutes your surface will be completely dry. I add a little drop of linseed oil to slow down this drying process and add shine.
Tempera paintings on the chalk ground