does not work
Sorry, I guess that is only used for setting custom data and not visual properties.

I think the only way to change the color of an individual entry in a combo box is the implement a custom QAbstractItemModel for the items you want to add, and use your model's QAbstractItemModel::data() method to return the colors you want using the Qt::ForegroundRole role. You should study the Qt Model / View architecture to learn about this.