Using QColor::isValid() it is easy to check if a color has been assigned a valid color value or not. On QML side, there seems to be no easy way to do the same. While QML does distinguish between invalid and valid colors (see option3 and option4) checking and even resetting color properties to their initial (invalid) state, seems not possible in the way it would be expected.
qmlscene output is:
qml: Checking if #000000 is invalid.
Declare a readonly color property that is not assigned a value. This can then be used for comparison. Note that assigning "" directly in the property definition produces an "Invalid property assignment: color expected" error.
Reconstruct color using Qt.rgba and check if the reconstructed color still compares to the original using Qt.colorEqual