Make the loop in the new yarn. (Make them in all the colors coming up to save time later.) Make it small but big enough to stick your hook through. Do NOT cut the tail off. You will use it to tighten the loop.
Pinch the place where the old color yarn is to change into the new color. Unravel about 10 inches of the work. On the other side of the pinch (toward the skein!) cut about 10 inches. Keep holding that pinch.
Stick the hook through the new color's loop. Take your pinched place in the old strand (don't let go, just loop it over the hook) and pull it through the new yarn's loop until the loose end has come all the way through.
Tighten the old color loop very tight on the pinched place; then make the new loop (new color) and tighten it.
Done carefully, this can give the impression that the yarn changed color at the exact spot it is supposed to. See the picture, where it is exactly at the end of the tan row and beginning of the green row.
