Topic: unselected image in grey-scale?

Hello everyone. I know that a similar thread was already discussed some time ago, but there was not really a solution. Is there any chance that the effect on the unselected image is more ajustable than just the brightness? My biggest wish would be that the unselected immages are just in grey-scale and turn to colour when selected. Does anyone have a clue. For help I would be very thankful!