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contrast, duh. Top square is fully surrounded by lighter tones/ color, therefore a higher contrast so it appears darker. The other square is set on the darker background, so the contrast isn't as significant, so it appears lighter.
webcams, duh. when your webcam sees you put your finger across the middle of the page, it sends a signal to the website hosting the image which then turns the image into a gif and morphs the two colours into the same colour. basic logic.