I think there are probably several reasons for saying the darker the better. If you set up in shallow water and like to sight fish - the darker the better. If you use an underwater camera - the darker the better. If you want it to absorb heat energy and keep it warmer - the darker the better. I don't know how much light passes through the dark green plastic tarps, but I would look at that. I've also seen somewhere a silver/black that I would check. If you want that for heat absorption, put the black side out.
Stay away from plaid!
