Stained Glass is a transparency type in the Transparency Tool. The effect is similar to viewing underlying objects through colored glass. It is useful for the simulation of glass and selectively darkening objects.

The color of the object with Stained Glass applied:

White - no effect on the colors of underlying objects.

Gray & Black - darkens the colors of underlying objects.

Other colors - darken underlying colors towards saturated color. (Light-red over light-red gives a mid-red: green over red gives black.) If you want to use a colored object to darken underlying colors, you may find Darken transparency type easier to use.

Technically the process is subtractive in RGB color space.