screens.rpy

default persistent.brightness = 1.0

screen preferences():

# Add these somewhere inside

label _("Brightness")
bar value VariableValue("persistent.brightness", 1.0)

script.rpy

image bg white = "#FFF"
image bg red = "#F00"

transform brightness_transform():
    matrixcolor BrightnessMatrix(persistent.brightness - 1.0)

init python:
    def brightness_changer(child, **properties):
        return At(child, brightness_transform())

init python:
    renpy.add_layer("background", above="master")
define config.tag_layer['bg'] = 'background'
define config.layer_transforms["background"] = [
    brightness_changer
]

label start:
    scene bg white
    "Is white now"
    scene bg red
    "Is red now"

    return