The complexity inside an interface should also be considered -- how the components of the 'bus' interact with each other with one connection to the outside worl

The complexity inside an interface should also be considered -- how the components of the 'bus' interact with each other with one connection to the outside world. If the 'bus' is simple enough to need few colors, then great. Same goes for the sibling comment regarding a ball of mud. It makes the analysis more complex but you should consider every global variable as connected to every other global in constructing the graph. Balls of mud, in my thinking, then need nearly as many colors as they have globals since every component theoretically interacts with every other. And when I mixed all the water colors as a kid, I was always disappointed to get a muddy color.