For custom widgets it is often needed to use extra colors, and these colors also to be consistent across the application.

QPalette fits this role in general, problem is one is limited to the ColorRole-s, predefined.
This is very limiting for "standard" custom widgets, and even worse for "non-standard" widgets, based on QGraphicsWidgest, where look'n'feel must also be consistent, but not necessarily (most often not) based on platform's visuals.

Considering QPalette is not passed by pointer, there is no way to fix the situation, except reinventing the wheel.