StainThe Stain shader is useful only when used in conjunction with other shaders. Its sole purpose is to define an area of effect: any material within the staining object is considered "stained". Different shaders make use of staining in various ways. (See Landscape and Wet shaders.)
Staining works by looking at the path of the rays: if a ray passes through a staining object, the ray becomes stained, and then stains any object it hits. This has an unexpected consequence: if the camera enters the staining object, eye rays will no longer strike the staining object before other surfaces, and stains will become "reversed." Try to avoid sending the camera into a staining object.
Create the staining object.
Make the staining object transparent.
In the Material dialogue box, select Stain as the material shader.