Hi, Have you checked under System Settings/Application Style/Window Decorations? That was at least the place to check until Plasma 5.11 . Wherever it is now, this KCM should give you a list of window decoration "themes", each of which can have configuration options. I don't see any option that affects the titlebar colour of the Breeze window theme directly, but maybe you are not using that theme without realising it? In the list I have installed here I see a "Pastel" theme that uses a horrible (IMHO) bright blue for the entire window frame.
It's not entirely impossible either that you're seeing some effect of Ubuntu branding - though blue seems incongruent for Canonical's usual colour palette. R.
