I disagree. I think that the OS should provide a set of UI elements so that all applications have a similar navigation and learnability. Colors however are a great way to set a Brand and distinguish apps from another. A benefit is that I can instantly see if I'm in Powerpoint(red) or Excel(green), even if the view I have is just some pixels wide (e.g. previews).

But i can definitely see the appeal of a system wide color scheme. Dark mode is a nudge in this direction.