I have read a million tutorials about this and can add a new color without problem. What NOBODY explains is what to do with the old color and how to fasten it off. Please help!!!
It sounds like you are joining the new color by crocheting over it for a bit before changing to it?
If you aren't, then I believe you will have to deal with the tail of the new yarn as well as the tail of the old yarn, i.e. hide them and fasten them off somehow.
Regardless, all yarn tails are usually best taken care of by weaving them in using a yarn tapestry needle after the granny square or project is done. The other alternative is to crochet over the old strand for several inches to fasten it off (however, this is not a very secure option. It's still best to weave in ALL ends.)
To weave in a yarn tail, thread it into a yarn tapestry needle. Then, weave the needle into your crochet fabric (in an area of the same color as the yarn tail) so as to hide the needle (and therefore the yarn) so it will not show on the right side of your work. This can be easy if you have a solid block of sc or other crochet stitches to hide inside. This will be a bit more challenging if you have a lacy crochet stitch, in which case you need to weave the tapestry needle through the more solid parts of the stitch to try to hide it.
Try to run the needle and yarn through the fabric for at least a few inches to really secure the yarn tail. This is more important if the yarn is at all slipperly (like a silk blend) of if the finished project will be subject to a lot of wear and tear. You really want to weave in enough to prevent the tail from worming itself out and then maybe causing a hole in your finished project.
Additionally, I like to weave the yarn in an inch or so in one direction, then another inch or so in a different direction. This will help prevent ends from working themselves out.
If you are working on a farily solid crochet fabric (like all sc fabric), then crocheting over the old yarn tail may be a good option. It just means to hold the old yarn in back of your work and work your next stitches as normal, but also around the old yarn. The old yarn will basically be encased inside these new stitches.
This is a good method only if the stitches really are close together and the hidden yarn color won't show through the stitches.
I think I might have over-answered you, but hopefully I've given you a complete answer!
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