Spraying is the quickest way to paint a complicated part evenly without obscuring detail, so it is the method of choice in the mass production of toys. To keep the paint spray from going where it is not wanted, a paint mask is made. This is a thin metal shell which conforms to the shape of the painted part. Holes are cut in the mask to reveal the areas of the part which are to receive paint. A separate mask is usually needed for each color used, and many multiples of each are needed to paint many objects at one time, but the singular term "paint mask" is usually used by Transformers fans to refer to the many actual pieces of metal used to produce one particular figure.
Once a paint mask has been produced, different colors can be easily sprayed through the same mask—it's only a matter of changing the color of the paint. The extra costs involved in this are negligible, amounting to paying the deco artist to pick colors. Changing the shape, number, or location of the paint application, however, requires a new set of masks.