Dyeing White and Grey Yarns with Brown Onion Skins

Currently, I'm interested in the effect of using plant material to overdye grey yarn, because although the plant dye itself must be just the same colour, it can look quite different on grey compared to white yarn. I've found it doesn't seem to matter whether the grey yarn is naturally a grey sheep colour or synthetically dyed. Since the latter is cheaper and easier to come by, my test subjects for brown onion skin dyeing were DROPS Alaska wool yarn in white and two shades of grey. 

As I huddled over the supermarket tray, picking out all the the loose onion skins, my companion, Elinor Gotland, got fidgety.
"At least put a couple of onions in the bag, Beaut. This is doing nothing for my image."
"I'm aiming for a really saturated dye colour, just give me a few minutes, it takes lots of onion skins to get a strong ginger." 
Next thing I knew, Elinor had shot across the aisle and appeared to be toying with the avocados as she turned with a smile and a gasp,
"Flossy! What a surprise!"

Once I had collected 100g brown onion skins, in order to extract the maximum dye, they were given a low boil for a full hour, were left in the pot to cool overnight and remained in the dye bath once the wool was added.  An equal weight of yarn was boiled for an hour, rather than simmered, and left to soak until the next day. Once dried, the results looked a proper ginger, with the light and dark grey yarns taking the colour toward brown. 
The fluid in the pot still looked yellow, though I'd done my best to get all the dye out of it.

A sample of the afterbath tested as mildly acidic at pH 5, possibly because by this stage, the onion skins had been fermenting in there for a couple of days. Adding vinegar to acidify another sample reduced the yellowness whereas adding soda ash turned it deep brown. Despite the obvious change to the dyebath, soaking either end of a length of dyed yarn in acid and alkali made minimal difference to the apparent dye colour on the wool. Casting about for an explanation, I turned to my companion.

"Maybe it's because that dyed ginger yarn is already at maximum strength, so reducing or increasing the colour slightly has too little effect to register."

Elinor was busy rearranging the yarn, her thoughts elsewhere.
"Let's just see how the dyed skeins look against the original wool colours."

"Hey, I need those undyed balls. Think I'll try dyeing another 100g of yarn."

There was still dye in the pot, though the second 100g of wool came out much paler. Grey yarn only shifted the gold toward a greenish beige. Probably not my most exciting discovery.

I tried soaking the pale gold yarn in the acid and alkali sample jars, but even then, there was minimal change in the colour at either end. I'm fairly confident there is no point in messing with the pH of an onion skin dyebath.

Elinor is contemplating a new image. At least if she becomes a redhead, I'll have no more fuss about collecting onion skins.