“In painting flats or units that are to be used in different scenes, under different colored lights, it is the common custom to paint a ground tone of neutral or almost neutral gray, either warm or cold in hue, and upon this spatter three or four or any number of different colors according to a definitely preconceived color scheme. The spattering colors may be primary colors of light or pigment, secondary colors of either, or any other chromatic system pleasing or displeasing to the artist. The value of these colors should be the same if there is not to be a predominance of any one color. This however, is arbitrary.
This technique results in a very flexible surface for lighting, and when properly combined with different hues and intensities of light it is capable of creating many imaginative and pleasing effects. If any one light hue is used alone on scenery it has a tendency to bring out that particular hue in the setting. If two colored lights are combined, the result will reflect its corresponding combination in pigment. Thus a red light upon a scene that has been spattered with red will produce a red glow to the audience; a combination of red and blue will bring out the red and blue qualities of the setting or their chromatic result, which is magenta.”
And here is an example of spatter on full-scale scenery for the Quincy, Illinois Scottish Rite Theatre (Secret Vault scene).