Why should we not wait. Is there anything making it urgent? FreeCAD is not a static background picture on the screen, it is a tool to work with, and that means to work with many different colors.
It doesn't make much sense to me to decide on the background color, and see later, that it collides with some other preferred color. As an example we can take the green color of a fully constrained sketch. Which I think we should keep. But if we decide on a green background, this has to be changed, and then people may realize that they prefer to keep the sketcher color over the new background.
You mentioned above the splash screen for 0.20, and there we decided too on the whole thing and not on certain sub-images.
To present a full color set doesn't mean that changes to single colors wouldn't be possible; which again would be similar to process for the splash screen.