From Ren'Py

Simple Flash Effect

This is the version of the effect that was used in Ori, Ochi, Onoe. It's designed to work with Ren'Py 5.6.1 and later.

This effect can be used for transitions such as flashbacks, light saturation effects and much more.

init:
    image bg road = "road.jpg"

    $ flash = Fade(.25, 0, .75, color="#fff")

label start:
    scene black # Or whatever image you're transitioning from.

    "We're about to change the background in a flash."

    scene bg road
    with flash

    "Wowsers!"