The color of the object really doesn't play a part as much as one would think. The only color that can be hard to find is white. It stands out well against the sea, but in any white cap condition, it is immediatley lost in the waves. In the night time, that also applies for any light shade of yellow, gray, lighter green and even some reds as they all look the same through night vision goggles in the dark.
Any type of refective material or tape works exceptionally well, especially on the bottom of an overturned boat hull. Also, a bright strobe style light that can be turned on only in an emergency, either on the boat or individual lifejacket, is invaluable.
....than again, ours are orange.