Accessing C++ Element exposed for QML in C++

I want to create a new component which can be exposed to QML.
I figured out that the following lines would do the job:

main.cpp

Code:

---

int main(int argc, char **argv) {
    ...
    Access access;
    ctxt->setContextProperty("access", &access);
    qmlRegisterType<TrayPaint>("TrayPaint", 1, 0, "TrayPaint");
    ...
}

---

As shown above there is an object “access” which is also exposed to QML.
How can I implement that the object “access” can change properties or call functions of the instance of object “TrayPaint” used in QML?

I thought on something like the objectName-Property in QML. But I’m not shure.

The TrayPaint object should display a (dynamic) number of circles. Each circles gets a number and a color property. The color should be changeable from C++.

I started with this tutorial.

like you would do in c++ :
access must know what instance of traypaint to interact with (a pointer).
and have functions declared as Q_INVOKABLE so that they can be called from qml.
from the qml side the instance pointer would be the id