Light determines the best color. Years ago, scientists dissected the eyes of walleyes and counted their color rods. They determined they could see blues, greens, yellows, oranges, and reds. Blues, greens and yellows are best used during bright sunlight and clear water conditions. Red is only red in shallow water. It is the first color to disappear as it gets deeper into the water column and it literally turns black..
Fluorescent yellow and fluorescent orange are best during low light and dingy water conditions. Black is not a color--it is the absence of color. Black is most visible to a fish as it is looking up. Think about it--without exception, all species have white bellies and dark backs. That way, because of their white bellies, they are less visible when their predators are below them and with their dark backs, they are less visible when their predators are looking down which is their basic camouflage
Since fish generally look for prey above them, including walleyes, black then becomes very visible and therefore, a good choice.
Footnote: I never use black leaders. I want the fish to see my lure not my leader.