Bleach is a transparency type in the Transparency Tool. Bleach has no clear analogy in the physical world. It is useful for highlights when the light source is not white.

The color of the object with Bleach applied:

Gray & White - lightens the colors in underlying objects.

Black - no effect on the colors of underlying objects.

Other colors - lighten underlying colors towards unsaturated color. (Light-red over light-red gives a pale-red: green over red gives yellow.) If you want to use a colored object to lighten but not color-shift underlying colors, you may find Lighten transparency type easier to use.

Technically the process is additive in RGB color space.