how to change visualized material transmissivity

I'm looking to produce different visualizations where I change the color and transmissivity of my window panes (see attached two images with the same color and different transmissivity).

Changing color is easy with a custom material (I'm using Trans in the radiance library), but when I change transmissivity I only see the decrease in incoming light across the room, but the "brightness" of the window doesn't change. In theory the window should appear darker as less light is transmitted through it.

Any fixes? Happy to do whatever method is easiest.

Thanks!