Using Opaque Watercolor
I would suggest the same range of colors, red, pale yellow and two blues, umber with the addition of white, as I did for the transparent range. The mixtures will be the same.
The difference will be in the constant changing of your water, the need to keep your palette constantly cleaned, and the use of white to lighten your tones. Poster colors are inclined to get muddy if you overmix them on your palette. Keep your mixtures simple. Don’t mix more than two colors and white together when trying them out. Be circumspect in adding a further color, for subtle tints. If your palette gets dirty, clean it. Don’t let the paint dry hard on the palette. Poster, egg tempera and casein don’t dissolve easily once they have dried hard. Keep your tubes and pots well covered when not in use. Using dried up paint has the added disadvantage of losing some of its binding strength; it works up too quickly when overpainting, often cracking or rubbing off even in thin coats.
Cardboard, or better still hardboard, the type that is smooth one side and rough the other, is a good panel on which to put your priming. The rough side, because of the regularity of the fine texture, will hold the priming perfectly. You can size the hardboard first, but as hardboard contains size already, this need not be done. Brush your coats of gesso evenly, give at least five coats. Let each coat dry first before applying the next coat. Keep your gesso thin and pliable. After you are satisfied that the board is well and truly covered, sand paper the final coat when dry to a smooth finish with a fine sand paper. (This form of priming will also prime canvas or board for oil painting.)
Using Egg Tempera
Egg tempera dissolves easily in water. It can be used in washes, and overpainted with white or with colors containing white. As the ground is absorbent, you won’t be able to cover as much area as you would with paper. The process of using tempera on a gesso ground is a slower procedure altogether. The Italian painters of the early Renaissance built up their pictures in carefully crosshatched strokes. Modern exponents of tempera, like Edward Wadsworth, do this but it isn’t necessary. Egg tempera can be used like transparent watercolor with the advantage of being suitable for overpainting.
Any type of brush may be used, but sable is preferable. A white plate or or white enameled palette is best to mix your colors on. Avoid wood palettes or any surface that is absorbent. If you are using your own made-up egg medium, care should be taken to mix the right proportion of medium to paint. Too much egg will result in flaking. Scrape the surface of your painting with a razor blade to ensure that you are not using too much egg. If it comes off in greasy, sticky shavings you are. This sort of thing does not occur with the ready-made-up colors.
Egg tempera can be used on any sort of paper or cardboard but you cannot build the paint into thick impastos as you can do with oil or casein. It is a most permanent medium, one of the most permanent of all, if carefully used.