In the games industry, I worked with an older programmer that developed games on the Sega Genesis back in the 90s. He said often changing the background colour[1] was the "simplest" way of debugging state or branches.
In my youth, us Amiga demoscene coders used this method too. You could basically measure CPU consumption by some sub-routine by setting a particular color on entry, then resetting on exit. Basically "raster lines" became a measure of time.
Pretty sure I optimized by putting a piece of tape at the exit line on the TV, then trying to make the color bar more narrow. :)
This can even be used for profiling on some systems because the background color can be changed in the middle of a frame with a single register write. So you can basically turn the left/right borders into a color-coded CPU utilization gauge.
[1] https://segaretro.org/Sega_Mega_Drive/Palettes_and_CRAM#Back...