A water color is a type of pigment that is mixed with water and creates a transparent paint.  It is a traditional medium for theatrical rendering.  The reason it is used in theatre is it provides the sketches with a luminescent quality that is similar to the luminescent quality of appearance of what costumes and scenery have when they are under the stage lights.  The watercolor becomes opaque if too much pigment is added to the mix.  When this happens the dried surface of the painting will have an uneven gloss, and the luminescent quality of the rendering will be lost.

There are three types of watercolor pigments used in theatrical rendering.  They are tube, cake, and liquid.  The tube colors are emulsified pigments that have about the same consistency of well-chilled sour cream.  Cake colors are manufactured as large hard blocks of watercolor pigment.  Small bottles of highly saturated hues are  how the liquid watercolors are packaged. 

The choice between tube and cake watercolors is a matter of personal preference because they both provide the same high-quality pigment.  When painting a large expanse, like a sky in a scenic rendering, tube colors are generally more convenient.  Cake colors are also a little bit more expensive if you only need a small amount of paint such as to provide a trim color on a costume sketch.  Dr. Martin's Watercolor's are a popular type of liquid watercolor.  They have an extremely strong saturation and brilliance.  These paints mix very easily, since they are already a liquid and they always remain transparent. 

Rendering with watercolor, designer's gouache, or acrylic paint will cause a lot fewer headaches if  a watercolor board is used instead of watercolor paper.  The board is more expensive but the backing prevents it from wrinkling and causes the paint to collect in little puddles.  Sometimes, though, using watercolor paper makes more sense.  Such as when 50 costume renderings need to be done for just a single show.  The weight of that portfolio makes the use of of watercolor paper a better choice.

Using watercolor paper requires a bit of preparation.  The paper should be mounted on some type of backing so it will not wrinkle when large washes are going to be made.  The paper should be thumb tacked or taped firmly  to a smooth board, such as a drawing board, or a smooth-finished plywood that is at least three-eighths of an inch thick.  After the paper is attached, and several hours before it is painted on, the paper should be thoroughly wetted and then let dry.  This "relaxes" the paper and lets it shrink before the paint is applied.

Cold-press illustration board, like Crescent 100 or Bristol board, or a rough-surface watercolor paper or board works very well for most scenic or costume renderings in watercolor. 



Source: Gillette, J. Michael. Theatrical Design and Production. 4th ed. Mountain View: Mayfield, 1999.