Denise asked how to weave in the threads when changing colors on a scarf. I agree with her that most sources are pretty sketchy about how to do that. Hiding the ends in a seam–the usual suggestion–is okay, but scarves don’t have seams.
Joelle Hoverson in her Last Minute Gifts suggests (on p.32) running the ends on the diagonal on the purl side, but doesn’t illustrate it. I like this method and it seems to work on scarves too when the weaving in is done with a light hand.
So here’s my attempt to illustrate.

I’ve used a red yarn to illustrate the technique, but there’s a strand of the background yarn three stitches to the right of the red.

Starting at the top and assuming I’m starting at the right edge of the fabric, using a darning needle with a bent tip, I take the yarn through the top of a downward loop (1), then through the top of the upward loop (2) immediately below and to the left of the first loop, pulling the yarn firmly, but not too tightly. Moving in the same direction, I take the yarn through the top of the downward loop (3) to the left of the second loop, pulling the yarn firmly but lightly so that it lays as flat as it can. After three or four loop passes to the left I start back (4) to the right finish back at the edge where I started. Usually I do not cut the remainder of the loose end until I have blocked the item and then I leave a bit (1/8 in.+) to allow for stretching. On an item that has an inside and an outside, like a hat, I leave an even longer loose end and fray it a bit.
When starting at the left edge, of course, you’d move first to the right and then the left.
The blue line shows where I’ve woven in the whitish yarn. You can see a slight ridge, but that is practically invisible at a moderate distance. Finger A shows where I cut the thread.
Finger B show a different method of weaving in, but it still follows the purl bumps and is almost invisible as well.
Here is the other side of the fabric.

The red barely shows; the white not at all.
Reader challenge: if you have a method that works for you, please post it. Weaving in ain’t that straightforward.