I use something called "Wooly Nylon" (it's a stretch nylon) to reinforce my socks that are 100% wool. It's virtually impossible to find the knitting "reinforcing yarn" to match colors unless it was dyed at the same time. There are zillions (give or take) of colors of wooly nylon. I don't know what it's used for exactly...it's used with serger sewing machines & this is where it can be found in the fabric store, by serger/sewing supplies, not in the knitting section. I've found it to be very helpful. Wish I could take the credit for this, of course...but I read about it from lots of other sock knitters

But, it's a matter of opinion if you need it with most sock yarn, to me, as misstailouise said, most sock yarn is already reinforced with nylon

. But, always look bc lots of handpainted sock yarns are 100% (hand wash, too) wool! have fun