This might sound strange to today's UI design crowd, by why is there so much emphasis on applications picking their own color palette when this should be the job of the operating system?
As a user I want a "coherent experience" when using my computing device, also when several apps are on screen at the same time. Your fancy app is not the center of the universe and doesn't "deserve" its own color palette. If I'm in a "green mood" today, I want to pick a green color palette for all apps, the same way I can switch to a different background image.
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.
As a user I want a "coherent experience" when using my computing device, also when several apps are on screen at the same time. Your fancy app is not the center of the universe and doesn't "deserve" its own color palette. If I'm in a "green mood" today, I want to pick a green color palette for all apps, the same way I can switch to a different background image.