When you need to change color or add more yarn, I have found a great transition that is barely noticeable. On the last step of a stitch, loop new yarn on hook and bring through last step. Crochet your stitches over the two loose ends for a neat and sturdy junction.
Step 1: Step 1
As you are finishing the last step of a stitch, like below is the last step of a single crochet. The last two loops are on the hook.