I recently bought some fine merino wool called Merinos 8 Shadow. It came with this Notice/Suggestion. “. . . . .special dye process that produces gentle shading and subtle variations . . . . . . To alleviate any possible striping, we suggest that you join 2 skeins of yarn and alternate them every 2 rows.” How exactly do I do this?
For some yarns, there is natural variation eve within dye lots because of the dyeing process. That's when they recommend that you alternate from 2 balls of yarn throughout so you won't don't end up with a project that has sections that look like different shades of the same yarn.
To alternate yarn every 2 rows, use the same technique you'd use to add a stripe to your project.
For instance, begin knitting or crocheting with ball #1. Work 2 rows of your project. Then, join in the 2nd ball at the end or Row 2/beginning of Row 3 and work Rows 3 and 4. You can join by just picking up the strand from ball #2 and using it to knit or crochet. It will feel "loose" because you have not knotted it to anything, but just keep using it and you can secure it later when you weave in all your loose ends at finishing time.
After Row 4, you should be on the same edge of your work where you left ball #1 hanging. Just drop the strand from ball #2 and use the strand from ball #1 to work the next 2 rows.
Repeat this last step, alternating balls #1 and #2 every 2 rows.
This method does assume you are either making a project where the "edges" of your work will end up in a seam, or the little bit of yarn that is carried over the 2 rows on each edge (whichever ball is unused for 2 rows) are not too noticeable.
Hope this answers it!
Of course, if you see no discernable difference in the shadings/colors of the balls of yarn you are using, then don't bother!