Are the plates grey? If they are then these clever photos are well taken. If they are white then you need to over expose like you would if you were photographing snow. Camera meters want to turn everything grey, indeed, if you had an old camera with a simple under/correct/over exposure meter, they way to check for correct exposure would be to use an 18% grey card, or in emergency, the palm of your hand to check the available light. If you try to photograph a very white scene without adjustment, the meter will under-expose the picture leading to grey whites. People often make the mistake of reducing the exposure when confronted with snow when in fact they should be over-exposing. Your plates, if indeed they are white, may be causing the greyness.
Edit:
Sorry, I'm repeating things as I've just read the post above mine.