Why was the result QColor(ARGB 1, 0, 0, 0)?

As I've said above, because you don't have access to the color in your for loop.
The series don't have any color by default and they are designed by the chart/chartview and its applied theme.
The default theme (0 or "QChart::ChartThemeLight") is what you see when you don't set any color.

Without making any further specifications, the slices' "own" brush is still the default one, which is ( 1, 0, 0, 0) (= 255, 0, 0, 0) in ARGB.
And (0, 0, 0) in RGB is... surprise... white.

@ntos said in How to save and restore QPen and QBrush states?:

Here is the whole code:

Great, then mark the topic as resolved please.

MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);

        QPieSeries* series = new QPieSeries();
        series->append("Vegetables", 0.4);
        series->append("Beans", 0.2);
        series->append("Fruit", 0.15);
        series->append("Seeds/Nuts", 0.1);
        series->append("Whole grains", 0.15);

        QPieSlice* slice0 = series->slices().at(0);
        slice0->setPen(QPen(Qt::darkMagenta, 2));
        slice0->setBrush(QBrush(Qt::magenta));
        QPieSlice* slice1 = series->slices().at(1);

        // set the pen to draw the border of the slice
        slice1->setPen(QPen(Qt::darkGreen, 2));
        // set the brush to brush the whole slice.
        slice1->setBrush(QBrush(Qt::green));
        QPieSlice* slice2 = series->slices().at(2);
        slice2->setPen(QPen(Qt::darkBlue, 2));
        slice2->setBrush(QBrush(Qt::blue));
        QPieSlice* slice3 = series->slices().at(3);
        slice3->setPen(QPen(Qt::darkYellow, 2));
        slice3->setBrush(QBrush(Qt::yellow));
        QPieSlice* slice4 = series->slices().at(4);
        slice4->setPen(QPen(Qt::darkCyan, 2));
        slice4->setBrush(QBrush(Qt::cyan));

        // Calculate the total value of the series
        qreal total = 0;
        for (const auto* slice : series->slices()) {
            total += slice->value();
        }



        for (auto* slice : series->slices()) {
            // Calculate percentage
            double percentage = (slice->value() / total) * 100;
            QBrush defaultBrush = slice->brush();
            QPen defaultPen = slice->pen();

           
            slice->setLabel(QString("%1 (%2%)").arg(slice->label()).arg(percentage, 1, 'f',1)); 
            slice->setLabelVisible();
            slice->setLabelPosition(QPieSlice::LabelInsideNormal);

            connect(slice, &QPieSlice::hovered, [slice, defaultBrush, defaultPen](bool hovered){
                if (hovered)
                {
                    slice->setPen(QPen(Qt::darkGreen, 2));
                    slice->setBrush(QBrush(Qt::green));
                    slice->setExploded(hovered);
                }
                else
                {
                    slice->setPen(defaultPen);
                    slice->setBrush(defaultBrush);
                    slice->setExploded(hovered);
                }

            });
        }
        QChart* chart = new QChart();
        chart->addSeries(series);
        chart->setTitle("What Derek ate this week");
        chart->legend()->hide();
        chart->setAnimationOptions(QChart::AllAnimations);

        QChartView* chartView = new QChartView(chart);
        chartView->setRenderHint(QPainter::Antialiasing);
        //set chartView as the main central widget
        this->setCentralWidget(chartView);


}

QPieSlice* slice2 = series->slices().at(2);
QBrush brush = slice2->brush();
qDebug() << "Default Brush is: " << pen;
for {  to change brush when mouse hovers over a slice and reuse brush when mouse moves away. } 

console output: Default Brush is: QBrush(QColor(ARGB 1, 0, 0, 0),NoBrush).
But as you see, I stored the brush before changing the slice properties, and the result was QColor(ARGB 1, 0, 0, 0). At this point, I hadn't hovered the mouse over the chart yet. Why was the result QColor(ARGB 1, 0, 0, 0)?
Further more, I didn't change any theme. This app is very simple. All it does is show a pie chart of 5 slices. I have no chance to change any theme.

Don't know why Qt devs decided on pen()/brush() returning a light grey colour instead of a slice's default one.

Don't blame Qt for your wrong code :)

If you add this before your connect, you will see that there is no data for defaultBrush.
Therefore it's just white

        qDebug() << "Default Brush is: " << defaultBrush;
        qDebug() << "Slice label: " << slicees->label();

Output:

Default Brush is: QBrush(QColor(ARGB 1, 0, 0, 0),NoBrush)
Slice label: "Jane"

It's even mentioned in the documentation:

By default, the visual appearance of the slice is set by a theme, but the theme can be overridden by specifying slice properties. However, if the theme is changed after the slices are customized, all customization will be lost.

( https://doc.qt.io/qt-6/qpieslice.html#details )

The "default" colors you see are the theme colors... the slice brush is still white.

The problem is when the mouse moves away, the colour of the slice is blank(white). Why?

Think about the scope of your local variables defaultPen and defaultBrush.
You capture them, but when the signal is emitted they are "default" default (which is white, I think), because the actual data from your slice cannot be accessed.