The Raster bar, also known as Copper bar, is a very popular graphics effect seen on many of the personal computers in the 80’s through 90’s era.
It was widely used in video games and especially graphics demos. Here is a link to the wikipedia article for more detail, https://en.wikipedia.org/wiki/Raster_bar.
It is accomplished by changing the background color in-between scan lines being drawn on the monitor/screen. This requires accessing the video controller hardware directly and changing appropriate registers as well as a way to time it precisely with the screen being drawn.