Not sure if this helps, but Yellow seems to hardest to obtain. For some strange reason, max you can get is half of what the other colors can give per fish. Not only that, but the chances of getting the yellow scales seem to be half of the other colors as well.
I didnt get a chance to fish for the full hour when trying yellow, but I barely got into the teens and didnt see any chance of getting more then the mid to high 20s in # of yellow scales if I did fish for the full hour. With the other colors, I usually average in the 70's once the hour is done. Friend of mine had also stated the same thing I've observed, but at the time, I just thought she was having bad luck with that particular color.