SUMMARY Changing the panel color is a trivial task in certain desktop environments (XFCE, MATE) and yet in KDE there is no way to do such a thing without having to manually edit certain files (color palette/panel's svg). It would have been great to have such functionality exposed to the user in a welcoming way.
We can't do without removing general themability, because Plasma themes affect far more than just the Panel's background color; they can use totally different graphics for it entirely.
Then is it impossible to introduce any way of doing those changes, at least for the default Plasma theme? Maybe some sort of "configuration" window that could generate a modified variant of the theme?
We're not planning to do that, sorry. There is an existing customization system for changing your Plasma appearance, and if we introduce too much visual customizability on top of it, we're undermining it and making it work less well.