If that's the primary purpose, we should go all black all the time. Is your contention that black full length pants are harder to differentiate, recognize and see by players wearing (white or non-white) pants that start at the hip and end just below the knee?
I don't think there is any great "visibility" difference experienced by players regarding the black pants versus white knickers, although I am contending that the standard uniform, consisting of white knickers AND black and white striped long stockings is more visible.
There are advantages, as well as disadvantages to both styles of pants. Full length pants allow for a wide variety of under garments to effectively deal with the weather, although those of us who work in extreme weather conditions have adapted to the options that are currently available, or choose to suffer. Weather resistant fabrics are a great development, but can be shaped to fit either style. I can't see any real advantage as for dealing with mud or slop as either way the garments require cleaning when soiled. As for knickers turning yellow, either they are not being washed properly or are being used well after their "expiration" date, which is a practice that will likely take the crispness out of black pants as well.
Knickers and striped socks have an advantage of being a recognizable icon associated ONLY with football officiating, world wide. Somewhere there's a lingering message about , "If it ain't broke, don't fix it".
