The "perfect" match is an illusion, but "close enough" looks perfect when the socks are on two separate feet.
First, let me say that I find exactly matching my Opal socks a fun challenge. For those who find it more frustrating than fun, I see nothing wrong with socks not matching if that is the knitter's intention.
Then there are some yarns with short color bursts where trying to get an exact match will only result in a headache: yarns like Opal Crocodile, Opal Handpainted, Opal Cool Ocean, Regia Line Steps.
For those yarns I just weight the skein on a kitchen scale, wind until half the skein is in a ball, cut the yarn and start winding a second ball.
That said, this is how I get the perfect match as displayed here in the pair of green Opal Magic socks I knit for my mother.
Take the band off the skein and weigh the skein on a kitchen scale.
Put the skein in a slippery bowl with tall sides or a wastebasket so it won't roll all over when winding. I usually wind from the outside of the skein.
Write the color changes down on a piece of paper as the yarn is wound so you can get a feel for the pattern and the repeat. Write whatever it takes so you can recognize the repeats.
It might look something like this:
- White with one inch black blobs
- Light green
- Light green with navy dots
- Dark green
Use the kitchen scale to determine when half the skein has been wound. Cut the yarn at the end of a color band.
Sometimes when I have a picture of a finished Opal sock, I know where I want the cuff to start, so I make sure that stripe is the last color wound.
The second ball wound is not likely to end at the same spot as the first. But if the socks are going to match, it's necessary to have it end at the same color band as the first ball. Frugal people will have a hard time with this, but there will be a little ball of waste yarn at the end of the second ball.
Once you have two balls, very carefully measure the distance from the caston knot to the end so the second sock is caston in exactly the same place.