But this does not work - the window gets a new color but it is no where near to the color I have set, nor the default color.
I would not say it doesn't work - doesn't work is when nothing happens.
But your code does change things, and even changes colors, just not the ones you thought will come out.
This can have various reasons, from an error on your part, to a visual effect created through translucency, where colors mix because they are translucent.

Hard to say.
Probably the key is in:
Since Qt 4.5, it has been possible to create windows with translucent regions on window systems that support compositing.
The question is if and how this feature is implemented in Symbian.

I have no experience on Symbian, maybe other forum members can help you more with Symbian specifics.