Why would you care about drop rates? Well the uncommon pigments are used for the more interesting recipes like off-hand frills and Darkmoon cards. If you plan to make any of those, you'll need uncommon pigments to do it. While this may not be an issue during the leveling process, towards the end when the nice items start to kick in you'll probably care. I conducted my test with Outland herbs - the best I could get my hands on from the Test Realm. Each herb had a least three stacks, maybe not a huge sampling but enough to see some trends. I noted what level the herb is (Herbalism required to harvest it), how many I milled, how many common (nether) pigments I got and how many uncommon (ebon) pigments I got. From this we should be able to calculate a rough drop rate for both pigment types.
|Herb (Skill)||Quantity||Nether Pigment||Ebon Pigment|
|Felweed (300)||170||81 (47.6%)||11 (6.5%)|
|Ragveil (325)||60||32 (53%)||6 (10%)|
|Ancient Lichen (340)||120||72 (60%)||14 (11.6%)|
|Netherbloom (350)||80||44 (55%)||12 (15%)|
|Nightmare Vine (365)||100||56 (56%)||14 (14%)|
As you can see, the drop rate of Ebon pigment was about twice as high on the better herbs than on the lower end herbs. My ragveil sample is a bit small, so it may just be off. Overall I see the drop rate for Ebon pigment going up as we move higher in the food chain. I also see the drop rate for Nether pigment increasing on high-end herbs by about 10%. My observations of the milling process support the calculations. When I was milling the netherbloom and nightmare vine, most of my milling actions resulted in an ebon pigment. But when I milled felweed, less than a quarter of my milling actions resulted in an ebon pigment. I also noticed that I was much more likely to get 3x or 4x nether pigment from a milling action with better herbs than I did with felweed, which tended to be mostly 2x with some 3x pigment drops.
In conclusion I believe the better herbs drop more uncommon pigment and slightly higher quantities of the common pigments. As I mentioned in the beginning, you probably won't care about this when you're powering up to 300 Inscription. Once you reach the Outland and Northrend herbs, it will start to matter how much uncommon pigment you get. Blizzard's concept of uncommonality seems to follow more closely to their Herbalism level charge than our in-game perceptions of rarity, so you might want to review their skill list when deciding what mix of herbs to stockpile.