have any of your birds got a red earlobe? The general rule is white ear white egg, red ear brown egg (though Penedesencas break it by having white earlobes [in a red ring] despite laying dark brown eggs). I know you said and showed most of them lay light brown eggs, but both the birds shown seem...
what breed is the 4th new hen? I'd wait till you are sure it's not her, before pursuing the hypothesis that an older hen has changed eggshell colour quite so dramatically as that photo suggests.