Why is middle lacquer black and top lacquer red?

The functional aspect of using different colors is that when you are applying your top red lacquer as a substrate for your metal, gold, silver, brass, etc., it is very easy to see what area has been applied to and what has not if you have a good contrast in color. If you were to apply red as your working middle lacquer and then use the same red as your substrate lacquer it would be very difficult to tell what has been covered and what hasn’t.