Change the solid circle color of interactive buttons.

Hi, you went in the right direction. Look at my NiceRadioButton item.
You define a new property at the level of the item that would the color:

RadioButton {
    id: control

    property var indicatorColor:  "black" // adds a new property

    // ...
    indicator: Rectangle {
        // ...
        Rectangle {
            // ...
            color: control.down ? "#555555" : control.indicatorColor  // use the new property
            visible: control.checked
        }
    }

And at the level of your GUI:

NiceRadioButton {
    indicatorColor: "red" // to obtain a radio buton which indicator is red
}

Rem: StackOverflow is good source of information for all these questions, better than this forum, because the guys and girls there have A LOT of knowledge about QML

By the way, what is doing your plugin ? Maybe does the same functionality already exist in Duration Editor plugin ? There is a Tuplet feature.