import QtQuick 2.0
import QtQuick.Controls 2.2

Item {
    property alias field: field

    TextField {
        id: field
        property alias backgroundColor: rectangle.color
        background: Rectangle {
            id: rectangle
            implicitWidth: 200
            implicitHeight: 40
            border.width: field.activeFocus ? 2 : 1
            color: field.palette.base
            border.color: field.activeFocus ? field.palette.highlight : field.palette.mid
        }
    }
}

import QtQuick 2.9

Item {
    Form {
        // This doesn't work:
        field.backgroundColor: "red"

        // This works:
        /*Component.onCompleted: {
            field.backgroundColor = "red";
        }*/
    }
}