You could do this on the Amstrad CPC (and many people did, mostly but not exclusively demo coders). The CPC had a palette of 27 possible colours, of which 16, 4, or 2 could be shown at once, depending on screen resolution (160px, 320px, 640px respectively, all x 200px). By reprogramming the palette selection on the fly, using two successive writes to output port &7F00, you could change the palette as the electron beam moved down the screen. You could even do it mid-screen row if your timing was good!