My window borders with adw-gtk3 (and a Gradience theme preset) on Budgie are not black/white but some sort of gray. So no, doesn't seem to be impossible in principle. Now what options are supported depends on your desktop environment, theme, the app's used toolkit, etc.
I do notice however that some parts of the theme seem to be stuck without logging out and back in here in this Budgie session (e.g. I got black window borders for some apps after switching widget themes, even after changing it back to adw-gtk3)