From milling thousands of Cataclysm herbs, I have discovered that Whiptail returns twice as much rare pigment as Cinderbloom.
Specifically, 100 mills will return (on average)
Herb RarePigments CommonPigments
Low Quality (CB) 25 250
High Quality (WT) 50 300
Similarly in Mists of Pandaria,
Fool's Cap (the only high quality herb) gives twice as much rare pigment as the other MoP herbs.
Unfortunately, due to the high cost of Fool's Cap on my server, I have only milled it 16 times, which is not good enough to show the difference. If you have Fool's Cap and want Misty Pigment, you'll get more Pigment if you sell the Fool's Cap and buy any other MoP herb that costs less than half the Fool's Cap. However, you will have to mill twice as many times - if you are time limited instead of gold limited, mill the Fool's Cap.