The QAbstractItemModel' data method failed to overwrite background

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I have a QTreeView which has a stylesheet applied to items with "QTreeView::item{background-color:#F7F7F7}". The view has a custom model and in the data method, I change the background of some specific cells like this :

if (index.column() == 3 && role == Qt::BackgroundColorRole)
{
    return QColor("#F4FF81");
}

if (index.column() == 3 && role == Qt::BackgroundColorRole)
{
    return QColor("#F4FF81");
}

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But this is not working to overwrite the value set in stylesheet. Is there anyone who can tell me what I miss? Thanks!!!