This is how hardware accelerated TV tuners worked back in the day, and probably also MPEG cards during their brief flash in the pan when they were necessary to play MPEG encoded video before processors were powerful enough to do it in software (and/or had various extensions added to them to assist, like MMX and SSE, etc., etc.).
I had an ATI TV Wonder card back in those dark days, and its mask color was hot magenta: RGB(255,0,255). Any pixels in your framebuffer of that color would be overwritten with TV output, although the player that came with the card already seemed to broadly know approximately where its output should be located so you couldn't relocate the video on your screen by doing this. If you full screened the player and then minimized it, though, you could color in any pixels on your display with e.g. Paint and they'd magically become little slices of broadcast television.