Change title color of QGroupBox only on parent

I have 2 QGroupBoxes and one of them is a child of the other. I want to change the color of the parent's title without changing the color on the child. I tried the following according to qt documentation: https://doc.qt.io/qt-5/stylesheet-examples.html
My code:
mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGroupBox>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();


private:
    Ui::MainWindow *ui;
    QGroupBox *r1;
    QGroupBox *r2;
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    r1 = new QGroupBox("test",this);
    r1->setGeometry(10,10,100,100);
    r1->setStyleSheet("QGroupBox::title#r1{color:red}");
    r1->show();
    r2 = new QGroupBox("test",r1);
    r2->setGeometry(10,10,80,80);
    r2->show();
}

MainWindow::~MainWindow()
{
    delete ui;
}

It doesn't work for me. The title does not change color.
If I do the same just by creating the QGroupBox in the designer it works. Help me figure out what I'm doing wrong?

Solution

The QSS slightly differs from CSS. Qt just do not know what is r1 in your case. To achieve your desired effect you can:

setObjectName() on the object you want to customize and then refer via this name. Therefore only that one element will have distinct color.

r1 = new QGroupBox("test",this);
r1->setGeometry(10,10,100,100);
r1->setObjectName("r1");
r1->setStyleSheet("QGroupBox#r1 {color: red}");
r1->show();
r2 = new QGroupBox("test",r1);
r2->setGeometry(10,10,80,80);
r2->show();

Use > selector to match parent - child relationship. In this case all QGroupBox that are direct children of QMainWindow will be red colored.

r1 = new QGroupBox("test",this);
r1->setGeometry(10,10,100,100);
r1->setStyleSheet("QMainWindow > QGroupBox {color: red}");
r1->show();
r2 = new QGroupBox("test",r1);
r2->setGeometry(10,10,80,80);
r2->show();

Use title property value - however, I would strongly not recommend that approach.

r1 = new QGroupBox("test",this);
r1->setGeometry(10,10,100,100);
r1->setStyleSheet("QGroupBox[title='test'] {color: red}");
r1->show();
r2 = new QGroupBox("test",r1);
r2->setGeometry(10,10,80,80);
r2->show();

More about syntax can be found here: https://doc.qt.io/Qt-5/stylesheet-syntax.html