Adding behavior prevents color change of rectangle

ocgltd

I have a simple rectangle (below) whose color depends on a function. It works great, changing between 3 colors depending on properties of the overall item.

In order to ease the transition between colors I added a behavior as shown below. However, once I add the behavior the color is frozen (does not change) when the variables change, and no error is shown.

What is wrong?

Rectangle {
    id: rectItemBackground
    color: backgroundColor()
    implicitWidth:  parent.width  -rectRightItemBackground.width/2
    implicitHeight: itemHeightByDepth(_depth)

    Behavior on color {
            NumberAnimation { duration: 1000 }
        }

}

function backgroundColor() {
    if (_selected) return gui.colorRGBString(GUIConstants.EColorElement_BackgroundSelected);
    if (_onSelectedPath) return gui.colorRGBString(GUIConstants.EColorElement_BackgroundPartialSelected);
    return gui.colorRGBString(GUIConstants.EColorElement_Background);
}

ocgltd

Seems to be an error in the docs (in an example). Once I used ColorAnimation instead of NumberAnimation it worked.