Below you will find short photo instruction how I change yarn color in the middle of the round or row. I used this method in my projects, e.g. cats in love or Amigurumi Kindergurumi (owl or reindeer).
yarn A = main yarn
yarn B = yarn in new color
1. Insert the hook in the last stitch (under both loops) and don't finish sc - leave two loops on the hook. Grab yarn B, leaving a tail:
2. Pull up yarn B through two loops on the hook:
3. Work sc in next st using yarn B. Secure the beginning of yarn B - link with yarn A or work three sc putting the hook under a tail of yarn B and yarn A:
4. Work sc in next stitches with yarn B. If you want to return to yarn A, don't finish the last stitch and leave two loops on the hook:
5. Grab yarn A (if you have cut it, leave a tail again, or if you haven't cut it, remember about proper tension to avoid deformation of your work) and pull up through remaining two loops (yarn B) on the hook:
6. Work sc in next st using yarn A. Secure the yarns as indicated above:
Sometimes I don't cut the yarn while changing color - if I change color on short area and I work in a round (on wrong side there are working yarns). But it is important to remember about proper tension because work can be easily deformated.