> Pitch black background, though perhaps it may have been modified by the bootleggers?
I'll post a summary, although this has been covered in depth elsewhere.
The cps1 video output has a hardware bug that causes the selected background color to be output when it's supposed to be outputting black for the monitor to calibrate the signal (known as reference black).
If a monitor is working within specification then it will get upset by this incorrect reference black and subtract red from the entire image, but not all monitors do work this way. It's believed on the balance of probabilities that the game was intended for monitors which don't look for the reference black at the time the cps1 board is incorrectly outputting the background color.
There is a hardware hack for cps1 boards to output the reference black at the correct time, so that all monitors will output the image in the same way.
In the case of that bootleg, it might either be that the roms have been hacked to change the background to black, it may be the video output doesn't support a background color, or the monitor might be processing the reference black in a way that is causing the red to stay in the image even though it's removed from the background.