I think that was also the idea behind the HAM(?) image format, that allowed you to display images with more than 256 colors, by dynamically changing the pallett

I think that was also the idea behind the HAM(?) image format, that allowed you to display images with more than 256 colors, by dynamically changing the pallette as the frame is drawn. A similar trick was used to change the location of the screen memory midway through the frame, and that's how they implemented the guru meditation error drop-down. Unfortunately, IIRC, the CPU was so slow the raster would travel about ten pixels between changes, so it was mostly just good for effects that used full scan lines, like raster bars, etc. You couldn't use it to programmatically draw a high resolution image or anything like that.