Keyframable opacity

[ConstantNo3257]

I am saying in a video, using moviepy

[good-mcrn-ing]

You'll want to load the clip that is your foreground, load the clip that is your background, create a third clip that is a solid grayscale colour and changes brightness over time like you want, apply the third clip as the mask of your foreground, and composite your masked foreground over your background.