There is a special relationship between foreground and background color that doesn't exist between any of the other things you mentioned. If one layer sets a f

There is a special relationship between foreground and background color that doesn't exist between any of the other things you mentioned. If one layer sets a foreground color, and another layer sets the background color to the same thing, the result is invisible text. That can happen with independently sane values for each. That cannot happen with any combinations of the other properties you mentioned.