Sempre, meaning "always," concerns the double-edged sword I have found this medium to be. Because you can apply paint layers endlessly, your painting can be in play always, never reaching a point beyond which you cannot go. Compare this to watercolor, which will go muddy and ugly if you try to paint too many layers or go back over an area too many times.
The wonderfully accommodating tempera medium, however, permits of innumerable new strokes, layers, etc. You can go back and forth between light and dark, opaque and transparent, warm and cool to your heart's content. This freedom, however, is both a great boon and a great bane, as you could tinker endlessly and never actually finish a painting.
Note to self: must stop temporizing with my current painting.