Casein and Poster Color
The advantages of using casein color are that it is a good paint for quick handling. It dries quickly and is able to stand nearly as much overpainting as oil.
In some ways it resembles oil in so far as it is possible to build up quite heavy impastos without the color cracking. A good casein should dry bone hard and withstand any effort to damage the surface.
Poster color, in the form of tubes or pots, is the last opaque watercolor that concerns us. The better varieties are usually called designers’ colors or gouache colors. Used exactly like transparent watercolor, with the advantage of being suitable for overpainting, they can be employed to paint nearly every subject. On paper or on card with sable brushes they have a great range of color and flexibility. The only disadvantage I can see is that you cannot paint large pictures with them. Anything over Imperial size tends to diminish its power. But for small scale pictures and outdoor sketching these colors are invaluable.
Opaque watercolors are a good stepping stone to oil painting as they have similar qualities and, except for drying mat instead of with a high gloss, are similar to handle. The color mixtures are similar too. Oil painting, however, needs more equipment and is not so easy to take outdoors, whereas a few tubes of poster color and a sketch book are.
It is still advisable, however, to use stretched paper or cardboard for painting on if you don’t want a cockled paper after completion.
You can use opaque color’s on top of or underneath a drawing. That is, you can draw first and obliterate the drawing. Or do your painting first, with or without a preparatory drawing, and draw any changes or developments on top. I do advise mixing your drawing media with your painting media. It makes the business of painting and drawing more of a piece and so gives you more leeway for error. If you confine yourself to too purist an attitude towards them by keeping them tightly separated into compartments you miss half the fun and so get a distorted idea of what it is all about. Because artists have kept these media in different departments in the past, there is no justification for you to do so as well. You have the advantage of a wider range of media that can be bought quite easily, ready for use.
The artists of the past didn’t have this advantage. They had to prepare all their colors themselves. Consequently, to save time and trouble, they often limited themselves to one or two media at the most-rarely moving from the one medium they practiced best. Oil painters, you will find, stick to oil paint, watercolor painters to watercolor and tempera to tempera. Of late, we find that the modern artist is not so insular and moves easily from watercolor to oil painting and on to etching and lithography, borrowing the technique of one medium for use in another.