when you want to change the color of your amigurumi or other crochet projects, always change color in the last 2 loops of a stitch, so the loop on the hook is the new color. The next stitch will be the first stitch with the new color.
  Let's change the yarn's color. We will do a single crochet. we insert the hook in the next stitch, and do a yarn over and through the hole.  
  We have 2 loops on the hook, now drop the current yarn, pick the other color yarn, put it on the wrong side, and pinch it with our middle finger and thumb. Do a yarn over and through the loops on the hook.
  See it again.…… Then we can crochet with new color yarn. we insert the hook in the next stitch, and do a single crochet.……