You could assign it a distinctive icon with QTabWidget::setTabIcon.

If you want to control individual tabs with more options than QTabWidget provides, sub-class it and use QTabWidget::tabBar to get the tab bar widget; that would allow you to change the text color, for example, of individual tabs.

It looks as if the tab bar itself is one widget, with no provision for changing the background color of individual tabs; so if you want to do that, I think you would have to implement a custom QTabBar::paintEvent using QTabBar::tabRect to figure out which areas need to be painted with a different background, and use QTabWidget::setTabBar to make the tab widget use your custom tab bar.