import QtQuick 2.4

Rectangle {
  width: 400
  height: 500

  anchors.fill: parent
  color: "red"

  Binding on color {
    when: ma.pressed
    value: ma.mouseX % 2 ? "green" : "blue"
  }

  MouseArea {
    id: ma
    anchors.fill: parent
  }
}

According Binding type docs:

In some cases you may want to modify the value of a property when a certain condition is met but leave it unmodified otherwise. Often, it's not possible to do this with direct bindings, as you have to supply values for all possible branches.
[...]
The Binding type restores any previously set direct bindings on the property.

Now I know that in my case it's not properly a binding, but a static value in fact setting color: { "red" } would make this to work, but I was expecting the binding to be completely set back to the "default" value (or to the component customer-set value) once invalid.