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.
Step 2: Step 2
With the two loops on the hook, wrap the new color or additional yarn around the hook.
Step 3: Step 3
Pull the new color or addition through the two loops of the stitch.
Step 4: Step 4
Now continue with the pattern, working over the two ends. This will hide them nicely.
Step 5: Step 5
Now continue with the single crochet and pull loop through.
Step 6: Step 6
Remember to make sure that you are working over the loose ends!
Step 7: Ending
As you can see by the pic below, I have worked single crochet over the loose ends. You can see the ends sticking out. Once you have crocheted over them enough to secure them, trim ends.